diff --git a/.build.ps1 b/.build.ps1
index baa79f6..366dd24 100644
--- a/.build.ps1
+++ b/.build.ps1
@@ -220,12 +220,22 @@ task PushNuGet NuGet, {
 Clean
 
 # Synopsis: Make and push the PSGallery package.
-task PushPSGallery Package, Version, {
-	assert ($TargetFramework -eq 'netstandard2.0')
-	$NuGetApiKey = Read-Host NuGetApiKey
-	Publish-Module -Path z/tools/$ModuleName -NuGetApiKey $NuGetApiKey
-},
-Clean
+task PushPSGallery @(
+	if ($TargetFramework -eq 'netstandard2.0') {
+		'Package'
+		'Version'
+		{
+			$NuGetApiKey = Read-Host NuGetApiKey
+			Publish-Module -Path z/tools/$ModuleName -NuGetApiKey $NuGetApiKey
+		}
+		'Clean'
+	}
+	else {
+		{
+			Invoke-Build PushPSGallery -TargetFramework netstandard2.0
+		}
+	}
+)
 
 # Synopsis: Test synopsis of each cmdlet and warn about unexpected.
 task TestHelpSynopsis {
diff --git a/Module/en-US/Mdbc.dll-Help.ps1 b/Module/en-US/Mdbc.dll-Help.ps1
index fd951ef..0caa642 100644
--- a/Module/en-US/Mdbc.dll-Help.ps1
+++ b/Module/en-US/Mdbc.dll-Help.ps1
@@ -312,7 +312,12 @@ $AClient = @{
 
 $ASession = @{
 	parameters = @{
-		Session = 'Specifies the client session which executes the command.'
+		Session = @'
+Specifies the client session which invokes the command.
+
+If it is omitted then the cmdlet is invoked in the current default session,
+either its own or the transaction session created by Use-MdbcTransaction.
+'@
 	}
 }
 
@@ -1156,3 +1161,56 @@ Tells to ignore document elements that do not match the properties.
 '@
 	}
 }
+
+### Use-MdbcTransaction
+Merge-Helps $AClient @{
+	command = 'Use-MdbcTransaction'
+	synopsis = 'Invokes the script with a transaction.'
+	description = @'
+The cmdlet starts a transaction session and invokes the specified script. The
+script calls data cmdlets and either succeeds or fails. The cmdlet commits or
+aborts the transaction accordingly.
+
+The transaction session is default for cmdlets with the parameter Session.
+For the script the session is exposed as the automatic variable $Session.
+
+Nested calls are allowed but transactions and sessions are independent.
+In particular, they may not see changes made in parent or nested calls.
+'@
+	parameters = @{
+		Script = @'
+Specifies the script to be invoked with a transaction session.
+'@
+	}
+
+	outputs = @(
+		@{
+			type = '[object]'
+			description = 'Output of the specified script.'
+		}
+	)
+
+	examples = @(
+		@{
+			code = {
+				# add several documents using a transaction
+				$documents = ...
+				Use-MdbcTransaction {
+					$documents | Add-MdbcData
+				}
+			}
+		}
+		@{
+			code = {
+				# move a document using a transaction
+				$c1 = Get-MdbcCollection MyData1
+				$c2 = Get-MdbcCollection MyData2
+				Use-MdbcTransaction {
+					# get and remove from MyData1 | add to MyData2
+					Get-MdbcData @{_id = 1} -Remove -Collection $c1 |
+					Add-MdbcData -Collection $c2
+				}
+			}
+		}
+	)
+}
diff --git a/Src/Commands/AbstractSessionCommand.cs b/Src/Commands/AbstractSessionCommand.cs
index 11cf428..142b959 100644
--- a/Src/Commands/AbstractSessionCommand.cs
+++ b/Src/Commands/AbstractSessionCommand.cs
@@ -2,9 +2,9 @@
 // Copyright (c) Roman Kuzmin
 // http://www.apache.org/licenses/LICENSE-2.0
 
-using MongoDB.Bson;
 using MongoDB.Driver;
 using System;
+using System.Collections.Generic;
 using System.Management.Automation;
 
 namespace Mdbc.Commands
@@ -15,6 +15,13 @@ public abstract class AbstractSessionCommand : Abstract, IDisposable
 		bool _disposed;
 		IClientSessionHandle _Session;
 
+		//! ThreadStatic and `= new Stack()` fails in Split-Pipeline
+		[ThreadStatic]
+		static Stack<IClientSessionHandle> _DefaultSessions_;
+		static Stack<IClientSessionHandle> DefaultSessions { get { return _DefaultSessions_ ?? (_DefaultSessions_ = new Stack<IClientSessionHandle>()); } }
+		internal static void PushDefaultSession(IClientSessionHandle session) { DefaultSessions.Push(session); }
+		internal static IClientSessionHandle PopDefaultSession() { return DefaultSessions.Pop(); }
+
 		[Parameter]
 		public IClientSessionHandle Session
 		{
@@ -22,8 +29,17 @@ public IClientSessionHandle Session
 			{
 				if (_Session == null)
 				{
-					_Session = MyClient.StartSession();
-					_dispose = true;
+					if (DefaultSessions.Count == 0)
+					{
+						// temporary session
+						_Session = MyClient.StartSession();
+						_dispose = true;
+					}
+					else
+					{
+						// the current default session
+						_Session = DefaultSessions.Peek();
+					}
 				}
 				return _Session;
 			}
diff --git a/Src/CollectionExt.cs b/Src/Commands/CollectionExt.cs
similarity index 97%
rename from Src/CollectionExt.cs
rename to Src/Commands/CollectionExt.cs
index 9ed6d7b..59c8a81 100644
--- a/Src/CollectionExt.cs
+++ b/Src/Commands/CollectionExt.cs
@@ -6,14 +6,14 @@
 using MongoDB.Driver;
 using System.Collections.Generic;
 
-namespace Mdbc
+namespace Mdbc.Commands
 {
 	static class CollectionExt
 	{
 		public static long MyCount(this IMongoCollection<BsonDocument> collection, IClientSessionHandle session, FilterDefinition<BsonDocument> filter, long skip, long first)
 		{
 			if (skip <= 0 && first <= 0)
-				return collection.CountDocuments(filter);
+				return collection.CountDocuments(session, filter);
 
 			var options = new CountOptions();
 			if (skip > 0)
diff --git a/Src/Commands/UseTransactionCommand.cs b/Src/Commands/UseTransactionCommand.cs
new file mode 100644
index 0000000..f8553b9
--- /dev/null
+++ b/Src/Commands/UseTransactionCommand.cs
@@ -0,0 +1,48 @@
+
+// Copyright (c) Roman Kuzmin
+// http://www.apache.org/licenses/LICENSE-2.0
+
+using MongoDB.Driver;
+using System;
+using System.Collections.Generic;
+using System.Management.Automation;
+
+namespace Mdbc.Commands
+{
+	[Cmdlet(VerbsOther.Use, "MdbcTransaction"), OutputType(typeof(IMongoDatabase))]
+	public sealed class UseTransactionCommand : AbstractClientCommand
+	{
+		[Parameter(Position = 0, Mandatory = true)]
+		public ScriptBlock Script { get; set; }
+
+		protected override void BeginProcessing()
+		{
+			var session = Client.StartSession();
+			AbstractSessionCommand.PushDefaultSession(session);
+			try
+			{
+				session.StartTransaction();
+				try
+				{
+					var vars = new List<PSVariable>() { new PSVariable("Session", session) };
+					var result = Script.InvokeWithContext(null, vars);
+					foreach (var item in result)
+						WriteObject(item);
+
+					session.CommitTransaction();
+				}
+				catch (RuntimeException exn)
+				{
+					var text = $"{exn.Message}{Environment.NewLine}{exn.ErrorRecord.InvocationInfo.PositionMessage}";
+					var exn2 = new RuntimeException(text, exn);
+					WriteException(exn2, null);
+				}
+			}
+			finally
+			{
+				AbstractSessionCommand.PopDefaultSession();
+				session.Dispose();
+			}
+		}
+	}
+}
diff --git a/Src/GlobalSuppressions.cs b/Src/GlobalSuppressions.cs
index e36d7c5..d2aca33 100644
--- a/Src/GlobalSuppressions.cs
+++ b/Src/GlobalSuppressions.cs
@@ -3,4 +3,4 @@
 [assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1819:Properties should not return arrays", Justification = "rk", Scope = "namespaceanddescendants", Target = "Mdbc.Commands")]
 [assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1825:Avoid zero-length array allocations.", Justification = "<Pending>", Scope = "member", Target = "~F:Mdbc.MyDocument.EmptyArray")]
 [assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1010:Collections should implement generic interface", Justification = "<Pending>", Scope = "type", Target = "~T:Mdbc.Collection")]
-[assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1010:Collections should implement generic interface", Justification = "<Pending>", Scope = "type", Target = "~T:Mdbc.Dictionary")]
+[assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1010:Collections should implement generic interface", Justification = "<Pending>", Scope = "type", Target = "~T:Mdbc.Dictionary")]
\ No newline at end of file
diff --git a/Src/Mdbc.csproj b/Src/Mdbc.csproj
index 7615bdf..ec58a34 100644
--- a/Src/Mdbc.csproj
+++ b/Src/Mdbc.csproj
@@ -16,7 +16,7 @@
     <PackageReference Include="PowerShellStandard.Library" Version="5.1.0" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="2.9.7">
+    <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="2.9.8">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
diff --git a/Tests/About.test.ps1 b/Tests/About.test.ps1
index 0dfd501..a245530 100644
--- a/Tests/About.test.ps1
+++ b/Tests/About.test.ps1
@@ -127,5 +127,6 @@ task PublicTypes {
 		'RenameCollectionCommand'
 		'SetDataCommand'
 		'UpdateDataCommand'
+		'UseTransactionCommand'
 	)
 }
diff --git a/Tests/MongoFiles.test.ps1 b/Tests/MongoFiles.test.ps1
index 9f2a33f..c633d0c 100644
--- a/Tests/MongoFiles.test.ps1
+++ b/Tests/MongoFiles.test.ps1
@@ -141,11 +141,11 @@ task Get-MongoFile Update-MongoFiles, {
 	$r = @(Get-MongoFile -CollectionName test 'collectionext|documentinput' | Sort-Object)
 	$r
 	equals 3 $r.Count
-	assert ($r[0] -clike '*\Mdbc\Src\CollectionExt.cs')
+	assert ($r[0] -clike '*\Mdbc\Src\Commands\CollectionExt.cs')
 	assert ($r[1] -clike '*\Mdbc\Src\DocumentInput.cs')
 	assert ($r[2] -clike '*\Mdbc\Tests\DocumentInput.test.ps1')
 
 	$r = @(Get-MongoFile -CollectionName test 'CollectionExt.cs' -Name)
 	equals 1 $r.Count
-	assert ($r[0] -clike '*\Mdbc\Src\CollectionExt.cs')
+	assert ($r[0] -clike '*\Mdbc\Src\Commands\CollectionExt.cs')
 }
diff --git a/Tests/Session.test.ps1 b/Tests/Session.test.ps1
deleted file mode 100644
index 936923b..0000000
--- a/Tests/Session.test.ps1
+++ /dev/null
@@ -1,27 +0,0 @@
-
-Import-Module Mdbc
-
-task Basic {
-	Connect-Mdbc -NewCollection
-	$ses = $Client.StartSession()
-	try {
-		@{_id = 71} | Add-MdbcData -Session $ses
-		$r = Get-MdbcData -Session $ses
-		equals $r._id 71
-
-		Set-MdbcData @{} @{p1 = 71} -Session $ses
-		$r = Get-MdbcData -Session $ses
-		equals $r.p1 71
-
-		Update-MdbcData @{} @{'$set' = @{p1 = 72}} -Session $ses
-		$r = Get-MdbcData -Session $ses
-		equals $r.p1 72
-
-		Remove-MdbcData @{} -Session $ses
-		$r = Get-MdbcData -Count -Session $ses
-		equals $r 0L
-	}
-	finally {
-		$ses.Dispose()
-	}
-}
diff --git a/Tests/Transaction.test.ps1 b/Tests/Transaction.test.ps1
new file mode 100644
index 0000000..232585c
--- /dev/null
+++ b/Tests/Transaction.test.ps1
@@ -0,0 +1,314 @@
+<#
+.Synopsis
+	Tests transactions and sessions.
+#>
+
+. ./Zoo.ps1
+
+# Synopsis: How to use manual and block transactions.
+task manual-and-block-transactions {
+	Connect-Mdbc . test
+	$Collection = Get-MdbcCollectionNew test
+
+	$session1 = $Client.StartSession()
+	try {
+		$session1.StartTransaction()
+		@{_id = 1} | Add-MdbcData -Session $session1
+		$session1.CommitTransaction()
+	}
+	finally {
+		$session1.Dispose()
+	}
+
+	$session1 = $Client.StartSession()
+	try {
+		$session1.StartTransaction()
+		@{_id = 2} | Add-MdbcData -Session $session1
+		$session1.AbortTransaction()
+	}
+	finally {
+		$session1.Dispose()
+	}
+
+	Use-MdbcTransaction {
+		@{_id = 3} | Add-MdbcData
+	}
+
+	Use-MdbcTransaction -ErrorAction 2 {
+		@{_id = 4} | Add-MdbcData
+		throw 'oops'
+	}
+
+	Get-MdbcData
+}
+
+# Synopsis: Move a document with a transaction.
+task move-document {
+	Connect-Mdbc . test
+	$c1 = Get-MdbcCollectionNew test1
+	$c2 = Get-MdbcCollectionNew test2
+	@{_id = 33} | Add-MdbcData -Collection $c1
+
+	Use-MdbcTransaction -ErrorAction 0 -ErrorVariable err {
+		Get-MdbcData @{_id = 33} -Remove -Collection $c1 |
+		Add-MdbcData -Collection $c2
+
+		equals (Get-MdbcData -Count -Collection $c1) 0L
+		equals "$(Get-MdbcData -Collection $c2)" '{ "_id" : 33 }'
+
+		throw 'oops'
+	}
+	assert ("$err" -like "oops*At*")
+	equals (Get-MdbcData -Count -Collection $c1) 1L
+	equals (Get-MdbcData -Count -Collection $c2) 0L
+
+	Use-MdbcTransaction -ErrorAction 0 -ErrorVariable err {
+		Get-MdbcData @{_id = 33} -Remove -Collection $c1 |
+		Add-MdbcData -Collection $c2
+
+		equals (Get-MdbcData -Count -Collection $c1) 0L
+		equals "$(Get-MdbcData -Collection $c2)" '{ "_id" : 33 }'
+	}
+	equals (Get-MdbcData -Count -Collection $c1) 0L
+	equals "$(Get-MdbcData -Collection $c2)" '{ "_id" : 33 }'
+
+	Remove-MdbcCollection test1
+	Remove-MdbcCollection test2
+}
+
+# Synopsis: Can use another session in a block transaction.
+task use-another-session-in-transaction {
+	Connect-Mdbc . test
+
+	$session1 = $Client.StartSession()
+	try {
+		$c1 = Get-MdbcCollectionNew test1
+		$c2 = Get-MdbcCollectionNew test2
+		@{_id = 33} | Add-MdbcData -Collection $c1
+
+		Use-MdbcTransaction {
+			# move the doc from one collection to another
+			Get-MdbcData @{_id = 33} -Remove -Collection $c1 |
+			Add-MdbcData -Collection $c2
+
+			# test in transaction session, moved
+			equals "$(Get-MdbcData -Collection $c1)" ''
+			equals "$(Get-MdbcData -Collection $c2)" '{ "_id" : 33 }'
+
+			# test in session 1, not moved
+			equals "$(Get-MdbcData -Collection $c1 -Session $session1)" '{ "_id" : 33 }'
+			equals "$(Get-MdbcData -Collection $c2 -Session $session1)" ''
+		}
+	}
+	finally {
+		$session1.Dispose()
+	}
+
+	Remove-MdbcCollection test1
+	Remove-MdbcCollection test2
+}
+
+# Synopsis: Show not just Use-MdbcTransaction position but the actual error, too.
+task show-inner-error-position {
+	Connect-Mdbc .
+	Use-MdbcTransaction -ErrorAction 2 -ErrorVariable err {
+		throw 'oops'
+	}
+	assert ("$err" -like "oops*At $BuildFile*throw 'oops'*")
+}
+
+# Synopsis: Transactions are not really nested.
+task nested-manual-transactions {
+	Connect-Mdbc . test test
+
+	function Test-Nested([switch]$Commit1, [switch]$Commit2) {
+		$Collection = Get-MdbcCollectionNew test
+		$session1 = $Client.StartSession()
+		try {
+			$session1.StartTransaction()
+			@{_id = 1} | Add-MdbcData -Session $session1
+
+			$session2 = $Client.StartSession()
+			try {
+				$session2.StartTransaction()
+				@{_id = 2} | Add-MdbcData -Session $session2
+
+				if ($Commit2) {
+					$session2.CommitTransaction()
+				}
+			}
+			finally {
+				$session2.Dispose()
+			}
+
+			if ($Commit1) {
+				$session1.CommitTransaction()
+			}
+		}
+		finally {
+			$session1.Dispose()
+		}
+	}
+
+	Test-Nested
+	equals "$(Get-MdbcData)" ''
+
+	Test-Nested -Commit1
+	equals "$(Get-MdbcData)" '{ "_id" : 1 }'
+
+	Test-Nested -Commit2
+	equals "$(Get-MdbcData)" '{ "_id" : 2 }'
+
+	Test-Nested -Commit1 -Commit2
+	equals "$(Get-MdbcData)" '{ "_id" : 1 } { "_id" : 2 }'
+}
+
+# Synopsis: Nested block transactions are allowed but they are independent.
+task nested-block-transactions {
+	Connect-Mdbc . test
+	$Collection = Get-MdbcCollectionNew test
+
+	# This function calls Use-MdbcTransaction with nested calls.
+	function Invoke-Job1 {
+		Use-MdbcTransaction {
+			# add data
+			@{_id = 1} | Add-MdbcData
+
+			# call nested, provide the current session if needed
+			Invoke-Job2 $Session
+
+			#! this session cannot see nested changes
+			$r = Get-MdbcData
+			equals "$r" '{ "_id" : 1 }'
+		}
+	}
+
+	#! Do not call the parameter "Session", Use-MdbcTransaction sets its own.
+	function Invoke-Job2($ParentSession) {
+		Use-MdbcTransaction {
+			#! this session cannot see parent changes without -Session
+			$r = Get-MdbcData
+			equals "$r" ''
+
+			# that is why, when needed, we use the session parameter
+			$r = Get-MdbcData -Session $ParentSession
+			equals "$r" '{ "_id" : 1 }'
+
+			# ok, add some data
+			@{_id = 2} | Add-MdbcData
+		}
+	}
+
+	Invoke-Job1
+	$r = Get-MdbcData
+	equals "$r" '{ "_id" : 1 } { "_id" : 2 }'
+}
+
+# Synopsis: Cannot start a session transaction when another is in progress.
+task start-transaction-twice {
+	Connect-Mdbc .
+	try {
+		$session1 = $Client.StartSession()
+		$session1.StartTransaction()
+		$session1.StartTransaction()
+		throw
+	}
+	catch {
+		$_
+		assert ("$_" -like '*Transaction already in progress.*')
+	}
+}
+
+# Synopsis: Simulate a conflict of two transactions.
+task conflicting-transactions {
+	# init counter
+	Connect-Mdbc . test test -NewCollection
+	@{_id = 1; n = 1} | Add-MdbcData
+
+	$session1 = $Client.StartSession()
+	$session2 = $Client.StartSession()
+	try {
+		$session1.StartTransaction()
+		$session2.StartTransaction()
+
+		# inc counter in 1
+		$r1 = Get-MdbcData -Session $session1 @{_id = 1} -Update @{'$inc' = @{n = 1}} -New
+		equals $r1.n 2
+
+		# inc counter in 2 -> error
+		try {
+			Get-MdbcData -Session $session2 @{_id = 1} -Update @{'$inc' = @{n = 1}} -New
+			throw
+		}
+		catch {
+			equals "$_" "Command findAndModify failed: WriteConflict."
+		}
+
+		# commit 2 -> error
+		try {
+			$session2.CommitTransaction()
+			throw
+		}
+		catch {
+			assert ("$_" -match 'Command commitTransaction failed: Transaction \d+ has been aborted')
+		}
+
+		# commit 1
+		$session1.CommitTransaction()
+	}
+	finally {
+		$session2.Dispose()
+		$session1.Dispose()
+	}
+
+	# inc worked once
+	$r = Get-MdbcData
+	equals "$r" '{ "_id" : 1, "n" : 2 }'
+}
+
+# Synopsis: Without transactions sessions see other changes.
+task sessions-without-transactions {
+	Connect-Mdbc -NewCollection
+	$session1 = $Client.StartSession()
+	$session2 = $Client.StartSession()
+	try {
+		@{_id = 1} | Add-MdbcData -Session $session1
+		@{_id = 2} | Add-MdbcData -Session $session2
+
+		$r = Get-MdbcData
+		equals "$r" '{ "_id" : 1 } { "_id" : 2 }'
+
+		$r = Get-MdbcData -Session $session1
+		equals "$r" '{ "_id" : 1 } { "_id" : 2 }'
+
+		$r = Get-MdbcData -Session $session2
+		equals "$r" '{ "_id" : 1 } { "_id" : 2 }'
+	}
+	finally {
+		$session2.Dispose()
+		$session1.Dispose()
+	}
+}
+
+# Synopsis: Script of Use-MdbcTransaction may output whatever.
+task block-output {
+	Connect-Mdbc . test
+	$Collection = Get-MdbcCollectionNew test
+
+	# nothing
+	$r = Use-MdbcTransaction {}
+	equals $r $null
+
+	# one object
+	$r = Use-MdbcTransaction {1}
+	equals $r 1
+
+	# many objects
+	$r = Use-MdbcTransaction {1; 2}
+	equals "$r" '1 2'
+
+	# null on abort, even if something was written before failure
+	$r = Use-MdbcTransaction -ErrorAction 0 -ErrorVariable err {1; throw 'oops'; 2}
+	equals $r $null
+	assert ("$err" -like "oops*throw 'oops'*")
+}
diff --git a/Tests/Zoo.ps1 b/Tests/Zoo.ps1
index 0b8b747..e2992c7 100644
--- a/Tests/Zoo.ps1
+++ b/Tests/Zoo.ps1
@@ -13,6 +13,13 @@ $ErrorPipeline = [Mdbc.Api]::TextParameterPipeline
 $ErrorSet = [Mdbc.Api]::TextParameterSet
 $ErrorUpdate = [Mdbc.Api]::TextParameterUpdate
 
+function Get-MdbcCollectionNew($Name, $Database=$Database) {
+	$Collection = Get-MdbcCollection $Name -NewCollection
+	Add-MdbcData @{_id = 1}
+	Remove-MdbcData @{}
+	$Collection
+}
+
 function Zoo1 {
 	class T { $Version = [version]'0.0' }
 	[T]::new()