diff --git a/lib/sessions.js b/lib/sessions.js
index 8d8920708..986c33dca 100644
--- a/lib/sessions.js
+++ b/lib/sessions.js
@@ -1,15 +1,37 @@
 'use strict';
 
-const retrieveBSON = require('./connection/utils').retrieveBSON,
-  EventEmitter = require('events'),
-  BSON = retrieveBSON(),
-  Binary = BSON.Binary,
-  uuidV4 = require('./utils').uuidV4;
+const retrieveBSON = require('./connection/utils').retrieveBSON;
+const EventEmitter = require('events');
+const BSON = retrieveBSON();
+const Binary = BSON.Binary;
+const uuidV4 = require('./utils').uuidV4;
+const MongoError = require('./error').MongoError;
 
-/**
- *
- */
+function assertAlive(session, callback) {
+  if (session.serverSession == null) {
+    const error = new MongoError('Cannot use a session that has ended');
+    if (typeof callback === 'function') {
+      return callback(error, null);
+    }
+
+    throw error;
+  }
+};
+
+/** A class representing a client session on the server */
 class ClientSession extends EventEmitter {
+
+  /**
+   * Create a client session.
+   * WARNING: not meant to be instantiated directly
+   *
+   * @param {Topology} topology The current client's topology
+   * @param {ServerSessionPool} sessionPool The server session pool
+   * @param {Object} [options] Optional settings
+   * @param {Boolean} [options.causalConsistency] Whether causal consistency should be enabled on this session
+   * @param {Boolean} [options.autoStartTransaction=false] When enabled this session automatically starts a transaction with the provided defaultTransactionOptions.
+   * @param {Object} [options.defaultTransactionOptions] The default TransactionOptions to use for transactions started on this session.
+   */
   constructor(topology, sessionPool, options) {
     super();
 
@@ -42,10 +64,20 @@ class ClientSession extends EventEmitter {
 
     this.explicit = !!options.explicit;
     this.owner = options.owner;
+    this.transactionOptions = null;
+    this.defaultTransactionOptions = options.defaultTransactionOptions || {};
+
+    if (options.autoStartTransaction) {
+      this.startTransaction();
+    }
   }
 
   /**
+   * Ends this session on the server
    *
+   * @param {Object} [options] Optional settings
+   * @param {Boolean} [options.skipCommand] Skip sending the actual endSessions command to the server
+   * @param {Function} [callback] Optional callback for completion of this operation
    */
   endSession(options, callback) {
     if (typeof options === 'function') (callback = options), (options = {});
@@ -56,6 +88,10 @@ class ClientSession extends EventEmitter {
       return;
     }
 
+    if (this.serverSession && this.inTransaction()) {
+      this.abortTransaction();  // pass in callback?
+    }
+
     if (!options.skipCommand) {
       // send the `endSessions` command
       this.topology.endSessions(this.id);
@@ -98,6 +134,98 @@ class ClientSession extends EventEmitter {
 
     return this.id.id.buffer.equals(session.id.id.buffer);
   }
+
+  /**
+   * @returns whether this session is current in a transaction or not
+   */
+  inTransaction() {
+    return this.transactionOptions != null;
+  }
+
+  /**
+   * Starts a new transaction with the given options.
+   *
+   * @param {Object} options Optional settings
+   * @param {ReadConcern} [options.readConcern] The readConcern to use for this transaction
+   * @param {WriteConcern} [options.writeConcern] The writeConcern to use for this transaction
+   */
+  startTransaction(options) {
+    assertAlive(this);
+    if (this.inTransaction()) {
+      throw new MongoError('Transaction already started');
+    }
+
+    // increment txnNumber and reset stmtId to zero.
+    this.serverSession.txnNumber += 1;
+    this.serverSession.stmtId = 0;
+
+    // set transaction options, we will use this to determine if we are in a transaction
+    this.transactionOptions = options || this.defaultTransactionOptions;
+  }
+
+  /**
+   * Commits the currently active transaction in this session.
+   *
+   * @param {Function} [callback] optional callback for completion of this operation
+   * @return {Promise} A promise is returned if no callback is provided
+   */
+  commitTransaction(callback) {
+    if (typeof callback === 'function') {
+      endTransaction(this, 'commitTransaction', callback);
+      return;
+    }
+
+    return new Promise((resolve, reject) => {
+      endTransaction(this, 'commitTransaction', (err, reply) => err ? reject(err) : resolve(reply));
+    });
+  }
+
+  /**
+   * Aborts the currently active transaction in this session.
+   *
+   * @param {Function} [callback] optional callback for completion of this operation
+   * @return {Promise} A promise is returned if no callback is provided
+   */
+  abortTransaction(callback) {
+    if (typeof callback === 'function') {
+      endTransaction(this, 'abortTransaction', callback);
+      return;
+    }
+
+    return new Promise((resolve, reject) => {
+      endTransaction(this, 'abortTransaction', (err, reply) => err ? reject(err) : resolve(reply));
+    });
+
+  }
+}
+
+function endTransaction(clientSession, commandName, callback) {
+  assertAlive(clientSession, callback);
+
+  if (!clientSession.inTransaction()) {
+    callback(new MongoError('No transaction started'));
+    return;
+  }
+
+  if (clientSession.serverSession.stmtId === 0) {
+    // The server transaction was never started.
+    callback(null, null);
+    return;
+  }
+
+  // send the command
+  clientSession.topology.command('admin.$cmd', { [commandName]: 1 }, {
+    writeConcern: clientSession.transactionOptions.writeConcern
+  }, (err, reply) => {
+    // reset internal transaction state
+    if (clientSession.options.autoStartTransaction) {
+      clientSession.startTransaction();
+    } else {
+      clientSession.transactionOptions = null;
+    }
+
+    callback(err, reply);
+  });
 }
 
 Object.defineProperty(ClientSession.prototype, 'id', {
diff --git a/lib/topologies/mongos.js b/lib/topologies/mongos.js
index 1ee863fb4..e7b3debc7 100644
--- a/lib/topologies/mongos.js
+++ b/lib/topologies/mongos.js
@@ -1,23 +1,22 @@
 'use strict';
 
-const inherits = require('util').inherits,
-  f = require('util').format,
-  EventEmitter = require('events').EventEmitter,
-  BasicCursor = require('../cursor'),
-  Logger = require('../connection/logger'),
-  retrieveBSON = require('../connection/utils').retrieveBSON,
-  MongoError = require('../error').MongoError,
-  errors = require('../error'),
-  Server = require('./server'),
-  clone = require('./shared').clone,
-  diff = require('./shared').diff,
-  cloneOptions = require('./shared').cloneOptions,
-  createClientInfo = require('./shared').createClientInfo,
-  SessionMixins = require('./shared').SessionMixins,
-  isRetryableWritesSupported = require('./shared').isRetryableWritesSupported,
-  getNextTransactionNumber = require('./shared').getNextTransactionNumber,
-  relayEvents = require('./shared').relayEvents;
-
+const inherits = require('util').inherits;
+const f = require('util').format;
+const EventEmitter = require('events').EventEmitter;
+const BasicCursor = require('../cursor');
+const Logger = require('../connection/logger');
+const retrieveBSON = require('../connection/utils').retrieveBSON;
+const MongoError = require('../error').MongoError;
+const errors = require('../error');
+const Server = require('./server');
+const clone = require('./shared').clone;
+const diff = require('./shared').diff;
+const cloneOptions = require('./shared').cloneOptions;
+const createClientInfo = require('./shared').createClientInfo;
+const SessionMixins = require('./shared').SessionMixins;
+const isRetryableWritesSupported = require('./shared').isRetryableWritesSupported;
+const incrementTransactionNumber = require('./shared').incrementTransactionNumber;
+const relayEvents = require('./shared').relayEvents;
 const BSON = retrieveBSON();
 
 /**
@@ -909,7 +908,7 @@ var executeWriteOperation = function(self, op, ns, ops, options, callback) {
   }
 
   // increment and assign txnNumber
-  options.txnNumber = getNextTransactionNumber(options.session);
+  incrementTransactionNumber(options.session);
 
   server[op](ns, ops, options, (err, result) => {
     if (!err) return callback(null, result);
diff --git a/lib/topologies/replset.js b/lib/topologies/replset.js
index 8d95dfa9b..8ae87c276 100644
--- a/lib/topologies/replset.js
+++ b/lib/topologies/replset.js
@@ -1,24 +1,24 @@
 'use strict';
 
-var inherits = require('util').inherits,
-  f = require('util').format,
-  EventEmitter = require('events').EventEmitter,
-  ReadPreference = require('./read_preference'),
-  BasicCursor = require('../cursor'),
-  retrieveBSON = require('../connection/utils').retrieveBSON,
-  Logger = require('../connection/logger'),
-  MongoError = require('../error').MongoError,
-  errors = require('../error'),
-  Server = require('./server'),
-  ReplSetState = require('./replset_state'),
-  clone = require('./shared').clone,
-  Timeout = require('./shared').Timeout,
-  Interval = require('./shared').Interval,
-  createClientInfo = require('./shared').createClientInfo,
-  SessionMixins = require('./shared').SessionMixins,
-  isRetryableWritesSupported = require('./shared').isRetryableWritesSupported,
-  getNextTransactionNumber = require('./shared').getNextTransactionNumber,
-  relayEvents = require('./shared').relayEvents;
+const inherits = require('util').inherits;
+const f = require('util').format;
+const EventEmitter = require('events').EventEmitter;
+const ReadPreference = require('./read_preference');
+const BasicCursor = require('../cursor');
+const retrieveBSON = require('../connection/utils').retrieveBSON;
+const Logger = require('../connection/logger');
+const MongoError = require('../error').MongoError;
+const errors = require('../error');
+const Server = require('./server');
+const ReplSetState = require('./replset_state');
+const clone = require('./shared').clone;
+const Timeout = require('./shared').Timeout;
+const Interval = require('./shared').Interval;
+const createClientInfo = require('./shared').createClientInfo;
+const SessionMixins = require('./shared').SessionMixins;
+const isRetryableWritesSupported = require('./shared').isRetryableWritesSupported;
+const incrementTransactionNumber = require('./shared').incrementTransactionNumber;
+const relayEvents = require('./shared').relayEvents;
 
 var MongoCR = require('../auth/mongocr'),
   X509 = require('../auth/x509'),
@@ -1230,7 +1230,7 @@ function executeWriteOperation(args, options, callback) {
 
   // increment and assign txnNumber
   if (willRetryWrite) {
-    options.txnNumber = getNextTransactionNumber(options.session);
+    incrementTransactionNumber(options.session);
   }
 
   return self.s.replicaSetState.primary[op](ns, ops, options, handler);
diff --git a/lib/topologies/shared.js b/lib/topologies/shared.js
index 7efab6904..6db60929d 100644
--- a/lib/topologies/shared.js
+++ b/lib/topologies/shared.js
@@ -425,9 +425,8 @@ const isRetryableWritesSupported = function(topology) {
  *
  * @param {ClientSession} session
  */
-const getNextTransactionNumber = function(session) {
+const incrementTransactionNumber = function(session) {
   session.serverSession.txnNumber++;
-  return BSON.Long.fromNumber(session.serverSession.txnNumber);
 };
 
 /**
@@ -454,5 +453,5 @@ module.exports.diff = diff;
 module.exports.Interval = Interval;
 module.exports.Timeout = Timeout;
 module.exports.isRetryableWritesSupported = isRetryableWritesSupported;
-module.exports.getNextTransactionNumber = getNextTransactionNumber;
+module.exports.incrementTransactionNumber = incrementTransactionNumber;
 module.exports.relayEvents = relayEvents;
diff --git a/lib/wireprotocol/3_2_support.js b/lib/wireprotocol/3_2_support.js
index 409fad7e7..6adb030f0 100644
--- a/lib/wireprotocol/3_2_support.js
+++ b/lib/wireprotocol/3_2_support.js
@@ -1,14 +1,14 @@
 'use strict';
 
-var Query = require('../connection/commands').Query,
-  retrieveBSON = require('../connection/utils').retrieveBSON,
-  f = require('util').format,
-  MongoError = require('../error').MongoError,
-  MongoNetworkError = require('../error').MongoNetworkError,
-  getReadPreference = require('./shared').getReadPreference;
+const Query = require('../connection/commands').Query;
+const retrieveBSON = require('../connection/utils').retrieveBSON;
+const f = require('util').format;
+const MongoError = require('../error').MongoError;
+const MongoNetworkError = require('../error').MongoNetworkError;
+const getReadPreference = require('./shared').getReadPreference;
 
-var BSON = retrieveBSON(),
-  Long = BSON.Long;
+const BSON = retrieveBSON();
+const Long = BSON.Long;
 
 var WireProtocol = function(legacyWireProtocol) {
   this.legacyWireProtocol = legacyWireProtocol;
@@ -57,8 +57,20 @@ var executeWrite = function(pool, bson, type, opsField, ns, ops, options, callba
   }
 
   // optionally add a `txnNumber` if retryable writes are being attempted
-  if (typeof options.txnNumber !== 'undefined') {
-    writeCommand.txnNumber = options.txnNumber;
+  if (options.session && options.session.serverSession) {
+    const serverSession = options.session.serverSession;
+    if (serverSession.txnNumber) {
+      writeCommand.txnNumber = BSON.Long.fromNumber(serverSession.txnNumber);
+    }
+
+    if (typeof serverSession.stmtId !== 'undefined') {
+      writeCommand.stmtId = serverSession.stmtId;
+
+      if (serverSession.stmtId === 0) {
+        writeCommand.startTransaction = true;
+        writeCommand.autocommit = false;
+      }
+    }
   }
 
   // Options object