diff --git a/src/compile/mark/mark.ts b/src/compile/mark/mark.ts
index ff78106dfa..614c224a26 100644
--- a/src/compile/mark/mark.ts
+++ b/src/compile/mark/mark.ts
@@ -18,8 +18,7 @@ import {UnitModel} from '../unit';
 
 import {isArray} from 'vega-util';
 import {X, Y} from '../../channel';
-import {getFieldDef} from '../../encoding';
-import {field} from '../../fielddef';
+import {field, getFieldDef} from '../../fielddef';
 import {isSelectionDomain} from '../../scale';
 
 const markCompiler: {[type: string]: MarkCompiler} = {
@@ -131,7 +130,7 @@ function detailFields(model: UnitModel): string[] {
         });
       }
     } else {
-      const fieldDef = getFieldDef(encoding, channel);
+      const fieldDef = getFieldDef<string>(encoding[channel]);
       if (fieldDef && !fieldDef.aggregate) {
         details.push(field(fieldDef, {binSuffix: 'start'}));
       }
diff --git a/src/compile/mark/mixins.ts b/src/compile/mark/mixins.ts
index 401c9f423f..a93e93f1c0 100644
--- a/src/compile/mark/mixins.ts
+++ b/src/compile/mark/mixins.ts
@@ -8,7 +8,7 @@ import {UnitModel} from '../unit';
 import * as ref from './valueref';
 
 import {NONSPATIAL_SCALE_CHANNELS} from '../../channel';
-import {Condition, FieldDef, isFieldDef, isValueDef} from '../../fielddef';
+import {Condition, FieldDef, getFieldDef, isValueDef} from '../../fielddef';
 import {predicate} from '../selection/selection';
 
 export function color(model: UnitModel) {
@@ -63,6 +63,7 @@ export function nonPosition(channel: typeof NONSPATIAL_SCALE_CHANNELS[0], model:
  * Return a mixin that include a Vega production rule for a Vega-Lite conditional channel definition.
  * or a simple mixin if channel def has no condition.
  */
+// FIXME support ConditionFieldDef
 function wrapCondition(model: UnitModel, condition: Condition<any>, vgChannel: string, valueRef: VgValueRef): VgEncodeEntry {
   if (condition) {
     const {selection, value} = condition;
@@ -77,6 +78,7 @@ function wrapCondition(model: UnitModel, condition: Condition<any>, vgChannel: s
   }
 }
 
+// FIXME support ConditionFieldDef
 export function text(model: UnitModel, vgChannel: 'text' | 'tooltip' = 'text') {
   const channelDef = model.encoding[vgChannel];
   const valueRef = (vgChannel === 'tooltip' && !channelDef) ? undefined : ref.text(channelDef, model.config);
@@ -96,7 +98,7 @@ export function bandPosition(fieldDef: FieldDef<string>, channel: 'x'|'y', model
         [channel+'c']: ref.fieldRef(fieldDef, scaleName, {}, {band: 0.5})
       };
 
-      if (isFieldDef(model.encoding.size)) {
+      if (getFieldDef(model.encoding.size)) {
         log.warn(log.message.cannotUseSizeFieldWithBandSize(channel));
         // TODO: apply size to band and set scale range to some values between 0-1.
         // return {
diff --git a/src/compile/mark/valueref.ts b/src/compile/mark/valueref.ts
index 9b27f313c6..8392f7b010 100644
--- a/src/compile/mark/valueref.ts
+++ b/src/compile/mark/valueref.ts
@@ -93,6 +93,7 @@ export function midPoint(channel: Channel, channelDef: ChannelDef<string>, scale
 
   if (channelDef) {
     /* istanbul ignore else */
+
     if (isFieldDef(channelDef)) {
       if (isBinScale(scale.type)) {
         // Use middle only for x an y to place marks in the center between start and end of the bin range.
diff --git a/src/compile/model.ts b/src/compile/model.ts
index 05a126747c..57667f2884 100644
--- a/src/compile/model.ts
+++ b/src/compile/model.ts
@@ -3,7 +3,7 @@ import {Channel, COLUMN, isChannel, NonspatialScaleChannel, ScaleChannel, X} fro
 import {CellConfig, Config} from '../config';
 import {Data, DataSourceType, MAIN, RAW} from '../data';
 import {forEach, reduce} from '../encoding';
-import {ChannelDef, field, FieldDef, FieldRefOption, isFieldDef, isRepeatRef} from '../fielddef';
+import {ChannelDef, field, FieldDef, FieldRefOption, getFieldDef, isFieldDef, isRepeatRef} from '../fielddef';
 import {Legend} from '../legend';
 import {hasDiscreteDomain, Scale} from '../scale';
 import {SortField, SortOrder} from '../sort';
@@ -400,14 +400,19 @@ export abstract class ModelWithField extends Model {
 
   public reduceFieldDef<T, U>(f: (acc: U, fd: FieldDef<string>, c: Channel) => U, init: T, t?: any) {
     return reduce(this.getMapping(), (acc:U , cd: ChannelDef<string>, c: Channel) => {
-      return isFieldDef(cd) ? f(acc, cd, c) : acc;
+      const fieldDef = getFieldDef(cd);
+      if (fieldDef) {
+        return f(acc, fieldDef, c);
+      }
+      return acc;
     }, init, t);
   }
 
   public forEachFieldDef(f: (fd: FieldDef<string>, c: Channel) => void, t?: any) {
     forEach(this.getMapping(), (cd: ChannelDef<string>, c: Channel) => {
-      if (isFieldDef(cd)) {
-        f(cd, c);
+      const fieldDef = getFieldDef(cd);
+      if (fieldDef) {
+        f(fieldDef, c);
       }
     }, t);
   }
diff --git a/src/compile/unit.ts b/src/compile/unit.ts
index bb160c216b..a0ecc8c80c 100644
--- a/src/compile/unit.ts
+++ b/src/compile/unit.ts
@@ -3,7 +3,7 @@ import {Channel, NONSPATIAL_SCALE_CHANNELS, UNIT_CHANNELS, UNIT_SCALE_CHANNELS,
 import {CellConfig, Config} from '../config';
 import {Encoding, normalizeEncoding} from '../encoding';
 import * as vlEncoding from '../encoding'; // TODO: remove
-import {field, FieldDef, FieldRefOption, isFieldDef} from '../fielddef';
+import {field, FieldDef, FieldRefOption, getFieldDef, isConditionalDef, isFieldDef} from '../fielddef';
 import {Legend} from '../legend';
 import {FILL_STROKE_CONFIG, isMarkDef, Mark, MarkDef, TEXT as TEXT_MARK} from '../mark';
 import {defaultScaleConfig, Domain, hasDiscreteDomain, Scale} from '../scale';
@@ -156,10 +156,13 @@ export class UnitModel extends ModelWithField {
       if (isFieldDef(channelDef)) {
         fieldDef = channelDef;
         specifiedScale = channelDef.scale;
+      } else if (isConditionalDef(channelDef) && isFieldDef(channelDef.condition)) {
+        fieldDef = channelDef.condition;
+        specifiedScale = channelDef.condition.scale;
       } else if (channel === 'x') {
-        fieldDef = vlEncoding.getFieldDef(encoding, 'x2');
+        fieldDef = getFieldDef(encoding.x2);
       } else if (channel === 'y') {
-        fieldDef = vlEncoding.getFieldDef(encoding, 'y2');
+        fieldDef = getFieldDef(encoding.y2);
       }
 
       if (fieldDef) {
@@ -249,12 +252,15 @@ export class UnitModel extends ModelWithField {
   private initLegend(encoding: Encoding<string>): Dict<Legend> {
     return NONSPATIAL_SCALE_CHANNELS.reduce(function(_legend, channel) {
       const channelDef = encoding[channel];
-      if (isFieldDef(channelDef)) {
-        const legendSpec = channelDef.legend;
-        if (legendSpec !== null && legendSpec !== false) {
-          _legend[channel] = {...legendSpec};
+      if (channelDef) {
+        const legend = isFieldDef(channelDef) ? channelDef.legend :
+          (channelDef.condition && isFieldDef(channelDef.condition)) ? channelDef.condition.legend : null;
+
+        if (legend !== null && legend !== false) {
+          _legend[channel] = {...legend};
         }
       }
+
       return _legend;
     }, {});
   }
diff --git a/src/encoding.ts b/src/encoding.ts
index 62fe7c769c..107446e982 100644
--- a/src/encoding.ts
+++ b/src/encoding.ts
@@ -5,19 +5,21 @@ import {CompositeAggregate} from './compositemark';
 import {Facet} from './facet';
 import {
   ChannelDef,
+  Condition,
+  ConditionalLegendDef,
+  ConditionalTextDef,
   ConditionalValueDef,
   Field,
   FieldDef,
   isFieldDef,
   isValueDef,
-  LegendFieldDef,
   normalize,
+  normalizeFieldDef,
   OrderFieldDef,
-  PositionFieldDef,
-  TextFieldDef,
+  PositionDef,
   ValueDef
 } from './fielddef';
-import {Conditional, ConditionalLegendDef, ConditionalTextDef, normalizeFieldDef, PositionDef} from './fielddef';
+import {getFieldDef, hasConditionFieldDef} from './fielddef';
 import * as log from './log';
 import {Mark} from './mark';
 import {isArray, some} from './util';
@@ -110,23 +112,12 @@ export function channelHasField(encoding: EncodingWithFacet<Field>, channel: Cha
     if (isArray(channelDef)) {
       return some(channelDef, (fieldDef) => !!fieldDef.field);
     } else {
-      return isFieldDef(channelDef);
+      return isFieldDef(channelDef) || hasConditionFieldDef(channelDef);
     }
   }
   return false;
 }
 
-export function getFieldDef<T>(encoding: EncodingWithFacet<T>, channel: Channel): FieldDef<T> {
-  const channelDef = encoding[channel];
-  if (isArray(channelDef)) {
-    throw new Error('getFieldDef should be never used with detail or order when they have multiple fields');
-  } else if (isFieldDef(channelDef)) {
-      return channelDef;
-  }
-  // TODO: if hasConditionFieldDef
-
-  return undefined;
-}
 
 export function isAggregate(encoding: EncodingWithFacet<Field>) {
   return some(CHANNELS, (channel) => {
@@ -135,7 +126,8 @@ export function isAggregate(encoding: EncodingWithFacet<Field>) {
       if (isArray(channelDef)) {
         return some(channelDef, (fieldDef) => !!fieldDef.aggregate);
       } else {
-        return isFieldDef(channelDef) && !!channelDef.aggregate;
+        const fieldDef = getFieldDef(channelDef);
+        return fieldDef && !!fieldDef.aggregate;
       }
     }
     return false;
@@ -153,8 +145,8 @@ export function normalizeEncoding(encoding: Encoding<string>, mark: Mark): Encod
 
     // Drop line's size if the field is aggregated.
     if (channel === 'size' && mark === 'line') {
-      const channelDef = encoding[channel];
-      if (isFieldDef(channelDef) && channelDef.aggregate) {
+      const fieldDef = getFieldDef(encoding[channel]);
+      if (fieldDef && fieldDef.aggregate) {
         log.warn(log.message.incompatibleChannel(channel, mark, 'when the field is aggregated.'));
         return normalizedEncoding;
       }
@@ -200,6 +192,8 @@ export function fieldDefs(encoding: EncodingWithFacet<Field>): FieldDef<Field>[]
       (isArray(channelDef) ? channelDef : [channelDef]).forEach((def) => {
         if (isFieldDef(def)) {
           arr.push(def);
+        } else if (hasConditionFieldDef(def)) {
+          arr.push(def.condition);
         }
       });
     }
diff --git a/src/fielddef.ts b/src/fielddef.ts
index 8a80c6963c..79649ab2d6 100644
--- a/src/fielddef.ts
+++ b/src/fielddef.ts
@@ -46,7 +46,7 @@ export type Condition<T> = {
  *   ...
  * }
  */
-export type ConditionalFieldDef<F, V> = F & {condition?: Condition<V>};
+export type ConditionalFieldDef<F extends FieldDef<any>, V extends ValueDef<any>> = F & {condition?: Condition<V>};
 
 /**
  * A ValueDef with Condition<ValueDef | FieldDef>
@@ -55,7 +55,7 @@ export type ConditionalFieldDef<F, V> = F & {condition?: Condition<V>};
  *   value: ...,
  * }
  */
-export type ConditionalValueDef<F, V> = V & {condition?: Condition<F | V>};
+export type ConditionalValueDef<F extends FieldDef<any>, V extends ValueDef<any>> = V & {condition?: Condition<F | V>};
 
 /**
  * Reference to a repeated value.
@@ -173,7 +173,18 @@ export interface TextFieldDef<F> extends FieldDef<F> {
 
 export type ConditionalTextDef<F> = Conditional<TextFieldDef<F>, ValueDef<string|number|boolean>>;
 
-export type ChannelDef<F> = FieldDef<F> | ValueDef<any>;
+export type ChannelDef<F> = Conditional<FieldDef<F>, ValueDef<any>>;
+
+export function isConditionalDef<F>(channelDef: ChannelDef<F>): channelDef is Conditional<FieldDef<F>, ValueDef<any>> {
+  return !!channelDef && !!channelDef.condition;
+}
+
+/**
+ * Return if a channelDef is a ConditionalValueDef with ConditionFieldDef
+ */
+export function hasConditionFieldDef<F>(channelDef: ChannelDef<F>): channelDef is (ValueDef<any> & {condition: FieldDef<F>}) {
+  return !!channelDef && !!channelDef.condition && isFieldDef(channelDef.condition);
+}
 
 export function isFieldDef<F>(channelDef: ChannelDef<F>): channelDef is FieldDef<F> | PositionFieldDef<F> | LegendFieldDef<F, any> | OrderFieldDef<F> | TextFieldDef<F> {
   return !!channelDef && (!!channelDef['field'] || channelDef['aggregate'] === 'count');
@@ -296,13 +307,32 @@ export function defaultType(fieldDef: FieldDef<Field>, channel: Channel): Type {
   }
 }
 
+/**
+ * Returns the fieldDef -- either from the outer channelDef or from the condition of channelDef.
+ * @param channelDef
+ */
+export function getFieldDef<F>(channelDef: ChannelDef<F>): FieldDef<F> {
+  if (isFieldDef(channelDef)) {
+    return channelDef;
+  } else if (hasConditionFieldDef(channelDef)) {
+    return channelDef.condition;
+  }
+  return undefined;
+}
+
 /**
  * Convert type to full, lowercase type, or augment the fieldDef with a default type if missing.
  */
-export function normalize(channelDef: ChannelDef<string>, channel: Channel) {
+export function normalize(channelDef: ChannelDef<string>, channel: Channel): ChannelDef<any> {
   // If a fieldDef contains a field, we need type.
   if (isFieldDef(channelDef)) {
     return normalizeFieldDef(channelDef, channel);
+  } else if (hasConditionFieldDef(channelDef)) {
+    return {
+      ...channelDef,
+      // Need to cast as normalizeFieldDef normally return FieldDef, but here we know that it is definitely Condition<FieldDef>
+      condition: normalizeFieldDef(channelDef.condition, channel) as Condition<FieldDef<string>>
+    };
   }
   return channelDef;
 }
@@ -347,7 +377,6 @@ export function normalizeFieldDef(fieldDef: FieldDef<string>, channel: Channel)
     log.warn(warning);
   }
   return fieldDef;
-
 }
 
 export function normalizeBin(bin: Bin|boolean, channel: Channel) {
diff --git a/src/stack.ts b/src/stack.ts
index 85a4bd2796..f84cd21e96 100644
--- a/src/stack.ts
+++ b/src/stack.ts
@@ -3,7 +3,7 @@ import * as log from './log';
 import {SUM_OPS} from './aggregate';
 import {Channel, STACK_GROUP_CHANNELS, X, X2, Y, Y2} from './channel';
 import {channelHasField, Encoding, isAggregate} from './encoding';
-import {Field, FieldDef, isFieldDef, PositionFieldDef} from './fielddef';
+import {Field, FieldDef, getFieldDef, isFieldDef, PositionFieldDef} from './fielddef';
 import {AREA, BAR, CIRCLE, isMarkDef, LINE, Mark, MarkDef, POINT, RULE, SQUARE, TEXT, TICK} from './mark';
 import {ScaleType} from './scale';
 import {contains, isArray} from './util';
@@ -57,8 +57,9 @@ export function stack(m: Mark | MarkDef, encoding: Encoding<Field>, stackConfig:
   const stackBy = STACK_GROUP_CHANNELS.reduce((sc, channel) => {
     if (channelHasField(encoding, channel)) {
       const channelDef = encoding[channel];
-      (isArray(channelDef) ? channelDef : [channelDef]).forEach((fieldDef) => {
-        if (isFieldDef(fieldDef) && !fieldDef.aggregate) {
+      (isArray(channelDef) ? channelDef : [channelDef]).forEach((cDef) => {
+        const fieldDef = getFieldDef(cDef);
+        if (!fieldDef.aggregate) {
           sc.push({
             channel: channel,
             fieldDef: fieldDef