diff --git a/README.md b/README.md
index 512da6f..554f75d 100644
--- a/README.md
+++ b/README.md
@@ -42,7 +42,8 @@ With JVC, you'd write this spec file and save it to a file
       "operation": "split",
       "creates": "src_split_files",
       "source": "src",
-      "interval": "60"
+      "interval": "60",
+      "validate": [{ "comment": "expected 12 files", "test": "output.length === 12" }]
   }]
 }
 ```
@@ -53,6 +54,9 @@ jvc my-spec.jvc
 Yes, the JVC is longer, but I think many would agree it is easier to read
 and maintain.
 
+As you can see above, JVC can also `validate` your operations to ensure that
+the output assets are what you expect them to be.
+
 **As the number of media assets and operations grows, hand-crafted shell
 scripts with magical ffmpeg incantations become ever more inscrutable.**
 
@@ -143,6 +147,13 @@ Trim audio/video; crop a section of an asset, becomes a new asset.
 Here is a [long, complex example](docs/complex_example.md) that uses
 every operation.
 
+## JVC JavaScript Expressions
+There are actually two DSLs in JVC. One is the JSON format for JVC files,
+which tells jvc where the assets are and what to do with them.
+
+The second DSL is not really a full DSL, it's more a JavaScript context
+in which you can evaluate expressions.
+
 ## What's with the name?
 A cross between a javelin and an icicle? JSON and a miracle?
 Something with positive connotations?
diff --git a/docs/asset_refs.md b/docs/asset_refs.md
new file mode 100644
index 0000000..d914e2e
--- /dev/null
+++ b/docs/asset_refs.md
@@ -0,0 +1,29 @@
+# Asset References
+Most operations have a `source` parameter that indicates what asset is
+considered the primary input to that operation.
+
+The value of `source` is the name of an asset, either defined in `assets`
+or as the output of a previous operation (named in the `creates` block).
+
+## List Assets
+The `split` operation creates multiple assets under the same name, this
+is called a "list asset", in contrast to a "singular asset" that only
+refers to one file.
+
+If you want to reference a specific file within a list asset, use square
+bracket notation:
+```shell script
+some_list_asset[0]     # first sub-asset
+some_list_asset[-1]    # last sub-asset
+some_list_asset[1..3]  # 2nd, 3rd and 4th assets
+some_list_asset[..3]   # 1st, 2nd, 3rd and 4th assets
+some_list_asset[3..]   # 4th asset and everything after
+```
+
+## Concat Operation `sources`
+The `concat` operation does not take a `source` param (which would name
+one asset), it takes a `sources` array of asset names.
+
+The named assets can be either singular assets or list assets.
+
+`concat` flattens the list of assets and concatenates them all together.
diff --git a/docs/concepts.md b/docs/concepts.md
index e42ce45..bd489b1 100644
--- a/docs/concepts.md
+++ b/docs/concepts.md
@@ -32,14 +32,32 @@ Assets expose properties that can be referenced in operations. The properties cu
 * `height`: width of the video in pixels (video and image assets only)
 
 ## Operations
-Operations are transformations to perform on the inputs.
+Operations represent transformations to perform on the inputs, and validations
+to ensure correctness.
+
+### Operation Assets
+An operation requires one or more input assets. These assets are referenced
+using the `source` (or for `concat`, `sources`) parameter.
 
 An operation can produce one or more new assets, which can then be referenced in
 later operations.
 
-Most of the operation settings can be JavaScript expressions, for example:
+Learn more about [Asset References](asset_refs.md).
+
+### Operation Configuration
+Many of the operation settings can be JavaScript expressions, for example:
 
     "start": "someAsset.duration - 10"
 
 The above would set the `start` value to ten seconds before the end of `someAsset`.
 
+Learn more about [JavaScript expressions in JVC](jvc_js.md).
+
+### Operation Validation
+An operation may define validations to perform by defining a `validation` array.
+
+Each object in the array has properties `comment` (to describe the test) and
+`test` which is a JavaScript expression. If it evaluates to `true` then the
+test passes. If `false`, the test fails and the `comment` is printed.
+
+Learn more about [JavaScript expressions in JVC](jvc_js.md).
diff --git a/docs/jvc_js.md b/docs/jvc_js.md
new file mode 100644
index 0000000..e1418f3
--- /dev/null
+++ b/docs/jvc_js.md
@@ -0,0 +1,50 @@
+# JVC JavaScript Expressions
+JavaScript expressions are used in a couple of places in JVC.
+
+  * Operation configuration
+  * Operation validations
+
+## Operation Configuration
+When you're writing the JSON for an operation, many of the operation's
+config parameters can be JS expressions that will be evaluated
+when the operation is run.
+
+For example, say you wanted to use `trim` to just chop a video in half.
+
+The `start` and `end` parameters of the `trim` operation support JavaScript
+expressions, so you could write:
+```js
+{
+  "comment": "cut video at midpoint",
+  "operation": "trim",
+  "creates": "chopped"
+  "source": "some_vid",
+  "end": "source.duration / 2"
+}
+```
+or run:
+```shell script
+jtrim some_vid.mp4 output_vid.mp4 0 "source.duration / 2"
+```
+In the above, `source` is referring to the source asset, `some_vid`.
+
+When JVC loads an asset (like `some_vid`), or an operation creates a new asset,
+the asset metadata is read using `mediainfo`. The metadata is stored in an object
+that is then put into the JavaScript context using the variable name `source`.
+
+## Operation Validations
+Once an operation completes, the validations run to ensure that the output
+assets pass whatever tests you want to run.
+
+Each test is a JavaScript expressions, which is evaluated to a boolean value.
+If the expression is true, the test passes. If false, the test fails and the
+`comment` associated with the failing test is printed.
+
+## JavaScript Context
+What variables are available in the JS context? And what properties do they have?
+
+### Assets
+Assets can be defined in the JVC's `assets` array, or as the output of an
+operation.
+
+Assets are referenced by their asset name.
\ No newline at end of file
diff --git a/src/main/java/jvc/model/operation/JOperation.java b/src/main/java/jvc/model/operation/JOperation.java
index d2582dd..ff9c4fc 100644
--- a/src/main/java/jvc/model/operation/JOperation.java
+++ b/src/main/java/jvc/model/operation/JOperation.java
@@ -32,6 +32,7 @@ public abstract class JOperation {
 
     @Getter @Setter private String operation;
     @Getter @Setter private JsonNode creates;
+    @Getter @Setter private JValidation[] validate;
     @Getter @Setter private boolean noExec = false;
 
     @Getter @Setter private String comment;
diff --git a/src/main/java/jvc/model/operation/JValidation.java b/src/main/java/jvc/model/operation/JValidation.java
new file mode 100644
index 0000000..7acd458
--- /dev/null
+++ b/src/main/java/jvc/model/operation/JValidation.java
@@ -0,0 +1,18 @@
+package jvc.model.operation;
+
+import lombok.Getter;
+import lombok.Setter;
+import org.cobbzilla.util.javascript.JsEngine;
+
+import java.util.Map;
+
+public class JValidation {
+
+    @Getter @Setter private String comment;
+    @Getter @Setter private String test;
+
+    public boolean eval(Map<String, Object> ctx, JsEngine js) {
+        return js.evaluateBoolean(test, ctx);
+    }
+
+}