diff --git a/build.sbt b/build.sbt
index 541e739..381de67 100644
--- a/build.sbt
+++ b/build.sbt
@@ -30,6 +30,11 @@ val Deps = new {
     val Effect: ModuleID = "org.typelevel" %% "cats-effect" % "3.6.3"
   }
 
+  val Circe = new {
+    val Core: ModuleID   = "io.circe" %% "circe-core"   % "0.14.15"
+    val Parser: ModuleID = "io.circe" %% "circe-parser" % "0.14.15"
+  }
+
   val Gs = new {
     val Datagen: ModuleID = "gs" %% "gs-datagen-core-v0" % "0.3.3"
     val Uuid: ModuleID    = "gs" %% "gs-uuid-v0"         % "0.4.1"
@@ -50,7 +55,8 @@ lazy val `gs-predicate` = project
   .aggregate(
     `test-support`,
     api,
-    keyValue
+    keyValue,
+    json
   )
   .settings(noPublishSettings)
   .settings(name := s"${gsProjectName.value}-v${semVerMajor.value}")
@@ -107,3 +113,24 @@ lazy val keyValue = project
       Deps.Gs.Uuid
     )
   )
+
+/** JSON - Defines predicates that can match on JSON values.
+  */
+lazy val json = project
+  .in(file("modules/json"))
+  .dependsOn(`test-support` % "test->test")
+  .dependsOn(api)
+  .settings(sharedSettings)
+  .settings(testSettings)
+  .settings(
+    name := s"${gsProjectName.value}-json-v${semVerMajor.value}"
+  )
+  .settings(
+    libraryDependencies ++= Seq(
+      Deps.Cats.Core,
+      Deps.Cats.Effect,
+      Deps.Circe.Core,
+      Deps.Circe.Parser,
+      Deps.Gs.Uuid
+    )
+  )
diff --git a/docs/README.md b/docs/README.md
new file mode 100644
index 0000000..97ec101
--- /dev/null
+++ b/docs/README.md
@@ -0,0 +1,6 @@
+# Library Documentation
+
+- [Terminology](./terminology.md)
+- [Predicates: Standard](./predicates-standard.md)
+- [Predicates: Key-Value](./predicates-key-value.md)
+- [Predicates: JSON](#)
diff --git a/docs/predicates-json.md b/docs/predicates-json.md
new file mode 100644
index 0000000..c71a799
--- /dev/null
+++ b/docs/predicates-json.md
@@ -0,0 +1,187 @@
+# Predicates: JSON
+
+Refer to [Terminology](./terminology.md) for information about specific terms.
+
+[[_TOC_]]
+
+## Description
+
+These predicates are focused on reasoning about JSON values.
+
+## Input
+
+All key-value predicates accept a single input: some JSON value.
+
+## JSON Query Strings
+
+This library may query JSON values using query strings. These are deliberately
+_not_ powerful and are not intended for `jq`-like functionality.
+
+These query strings directly interact with the query system.
+
+### Empty Query
+
+An empty query string indicates that the predicate should address the value
+verbatim.
+
+#### Example: Plain String
+
+**Input**:
+
+This example will match.
+
+```json
+"some string"
+```
+
+**Predicate**:
+
+```json
+{
+    "predicate": "json-value-equals",
+    "parameters": {
+        "queryString": "",
+        "value": "some string"
+    }
+}
+```
+
+### Value (`.`)
+
+The `.` operator accesses a named value by key within the current object. This
+will not match if the current value is not an object.
+
+#### Example: Mismatched Types
+
+**Input**:
+
+This example will not match, because the value at `x` is a string, so no key can
+possibly be accessed through it.
+
+```json
+{ "x": "some string" }
+```
+
+**Predicate**:
+
+```json
+{
+    "predicate": "json-value-equals",
+    "parameters": {
+        "queryString": "x.y",
+        "value": "some string"
+    }
+}
+```
+
+### JSON Key
+
+Any text aside from `.` and the contents of `[]` is considered a key. These keys
+refer to named elements of the JSON structure.
+
+```json
+{
+    "x": { "y": { "z": 100 } }
+}
+```
+
+`x`, `y`, and `z` are all keys.
+
+### Array Operator
+
+The `[any|all|<index>]` syntax is used to reason about arrays:
+
+- `[any]` indicates that the predicate will match if _any_ of the selected
+  values based on traversing the array match.
+- `[all]` indicates that the predicate will match only if _all_ of the selected
+  values based on traversing the array match.
+- `[<index>]` indicates that the predicate will match only if the specified
+  index exists within the array and the selected value at that index matches.
+
+#### Example: Value Equals (Any)
+
+The following scenario will _match_:
+
+**Input**:
+
+```json
+{
+    "data": {
+        "things": [
+            { "x": 100, "y": 200 }
+        ]
+    }
+}
+```
+
+**Predicate**:
+
+```json
+{
+    "predicate": "json-value-equals",
+    "parameters": {
+        "queryString": "data.things[any].x",
+        "value": 100
+    }
+}
+```
+
+#### Example: Value Equals (All)
+
+The following scenario will _match_:
+
+**Input**:
+
+```json
+{
+    "data": {
+        "things": [
+            { "x": 100, "y": 200 },
+            { "x": 100, "y": 300 },
+            { "x": 100, "y": 400 }
+        ]
+    }
+}
+```
+
+**Predicate**:
+
+```json
+{
+    "predicate": "json-value-equals",
+    "parameters": {
+        "queryString": "data.things[all].x",
+        "value": 100
+    }
+}
+```
+
+#### Example: Value Equals (Index)
+
+The following scenario will _match_:
+
+**Input**:
+
+```json
+{
+    "data": {
+        "things": [
+            { "x": 100, "y": 200 },
+            { "x": 999, "y": 300 },
+            { "x": 100, "y": 400 }
+        ]
+    }
+}
+```
+
+**Predicate**:
+
+```json
+{
+    "predicate": "json-value-equals",
+    "parameters": {
+        "queryString": "data.things[1].x",
+        "value": 999
+    }
+}
+```
diff --git a/docs/predicates-key-value.md b/docs/predicates-key-value.md
new file mode 100644
index 0000000..b3be72f
--- /dev/null
+++ b/docs/predicates-key-value.md
@@ -0,0 +1,109 @@
+# Predicates: Key-Value
+
+Refer to [Terminology](./terminology.md) for information about specific terms.
+
+[[_TOC_]]
+
+## Description
+
+These predicates are focused on reasoning about keys and their associated
+(or not associated) values.
+
+## Input
+
+All key-value predicates accept a single input: some key-value provider. This
+is the state referenced by the predicates.
+
+This provider can answer two questions:
+
+- Does the given key exist?
+- What is the value associated with a given key?
+
+## Predicate: Key Exists
+
+Matches if the key-value provider contains the given key.
+
+## Predicate: Value Equals
+
+Given some key `K` and expected value `EV`, this predicate matches if:
+
+- There is some value `V` associated with `K`.
+- `V` is equal to `EV`.
+
+## Predicate: Value Not Equals
+
+Given some key `K` and unexpected value `UV`, this predicate matches if:
+
+- There is some value `V` associated with `K`.
+- `V` is not equal to `UV`.
+
+## Predicate: String Contains
+
+> [!important]
+> This predicate is only supported for string values.
+
+Given some key `K` and string `S`, this predicate matches if:
+
+- There is some value `V` associated with `K`.
+- `V` contains the substring `S`.
+
+### Examples
+
+| `S`     | `V`     | Result  |
+| ------- | ------- | ------- |
+| `""`    | `""`    | `match` |
+| `""`    | `"a"`   | `match` |
+| `"a"`   | `""`    | `miss`  |
+| `"a"`   | `"abc"` | `match` |
+| `"ab"`  | `"abc"` | `match` |
+| `"b"`   | `"abc"` | `match` |
+| `"bc"`  | `"abc"` | `match` |
+| `"c"`   | `"abc"` | `match` |
+| `"d"`   | `"abc"` | `miss`  |
+| `"abc"` | `"abc"` | `match` |
+
+## Predicate: String Starts With
+
+> [!important]
+> This predicate is only supported for string values.
+
+Given some key `K` and prefix `P`, this predicate matches if:
+
+- There is some value `V` associated with `K`.
+- `V` starts with the prefix `P`.
+
+### Examples
+
+| `P`     | `V`     | Result  |
+| ------- | ------- | ------- |
+| `""`    | `""`    | `match` |
+| `""`    | `"a"`   | `match` |
+| `"a"`   |  `""`   | `miss`  |
+| `"a"`   | `"abc"` | `match` |
+| `"ab"`  | `"abc"` | `match` |
+| `"b"`   | `"abc"` | `miss`  |
+| `"d"`   | `"abc"` | `miss`  |
+| `"abc"` | `"abc"` | `match` |
+
+## Predicate: String Ends With
+
+> [!important]
+> This predicate is only supported for string values.
+
+Given some key `K` and suffix `S`, this predicate matches if:
+
+- There is some value `V` associated with `K`.
+- `V` ends with the suffix `S`.
+
+### Examples
+
+| `S`     | `V`     | Result  |
+| ------- | ------- | ------- |
+| `""`    | `""`    | `match` |
+| `""`    | `"a"`   | `match` |
+| `"a"`   |  `""`   | `miss`  |
+| `"a"`   | `"abc"` | `miss`  |
+| `"bc"`  | `"abc"` | `match` |
+| `"c"`   | `"abc"` | `match` |
+| `"d"`   | `"abc"` | `miss`  |
+| `"abc"` | `"abc"` | `match` |
diff --git a/docs/predicates-standard.md b/docs/predicates-standard.md
new file mode 100644
index 0000000..349041f
--- /dev/null
+++ b/docs/predicates-standard.md
@@ -0,0 +1,57 @@
+# Predicates: Standard
+
+Refer to [Terminology](./terminology.md) for information about specific terms.
+
+[[_TOC_]]
+
+## And
+
+The `and` predicate calculates the logical AND of some list of predicates. For
+this predicate, a `match` corresponds to `true` and a `miss` corresponds to
+`false`.
+
+> Only match if all contained predicates match.
+
+### Case: Empty List
+
+The predicate always misses.
+
+Since there are no predicates, nothing can possibly match.
+
+### Case: Single Predicate
+
+Identiical to that single predicate.
+
+### Case: Multiple Predicates
+
+The logical AND of the predicate results.
+
+## Or
+
+The `or` predicate calculates the logical OR of some list of predicates. For
+this predicate, a `match` corresponds to `true` and a `miss` corresponds to
+`false`.
+
+> Only match if any contained predicate matches.
+
+### Case: Empty List
+
+The predicate always misses.
+
+Since there are no predicates, nothing can possibly match.
+
+### Case: Single Predicate
+
+Identiical to that single predicate.
+
+### Case: Multiple Predicates
+
+The logical OR of the predicate results.
+
+## True
+
+This predicate always matches.
+
+## False
+
+This predicate always misses.
diff --git a/docs/terminology.md b/docs/terminology.md
new file mode 100644
index 0000000..18cca40
--- /dev/null
+++ b/docs/terminology.md
@@ -0,0 +1,33 @@
+# Predicate Terminology
+
+[[_TOC_]]
+
+## Definition: Predicate
+
+Any function that accepts exactly one input and produces a
+[Predicate Result](#definition-predicate-result) (aka a Boolean output).
+
+Predicates are ways to express logical conditions.
+
+## Definition: Predicate Result
+
+The result of evaluating a [Predicate](#definition-predicate). Predicates may:
+
+- [Match](#definition-match)
+- [Miss](#definition-miss)
+
+## Definition: Match
+
+The term **match** is a specific [Result](#definition-predicate-result).
+Semantically, it means that the predicate ran successfully against the given
+input. The conditions within were met.
+
+If the predicate were expressed as a Boolean, this would correspond to `true`.
+
+## Definition: Miss
+
+The term **miss** is a specific [Result](#definition-predicate-result).
+Semantically, it means that the predicate did not run successfully against the
+given input. The conditions within were not met.
+
+If the predicate were expressed as a Boolean, this would correspond to `false`.
diff --git a/modules/json/src/main/scala/gs/predicate/v0/json/JsonKeyExists.scala b/modules/json/src/main/scala/gs/predicate/v0/json/JsonKeyExists.scala
new file mode 100644
index 0000000..7eb9ae1
--- /dev/null
+++ b/modules/json/src/main/scala/gs/predicate/v0/json/JsonKeyExists.scala
@@ -0,0 +1,32 @@
+package gs.predicate.v0.json
+
+import cats.Applicative
+import gs.predicate.v0.api.Predicate
+import gs.uuid.v0.UUID
+import io.circe.Json
+
+/** Predicate that matches if JSON blob is an object that contains the given
+  * key.
+  *
+  * @param id
+  *   The unique identifier of this [[Predicate]].
+  * @param key
+  *   The key that should exist.
+  */
+final class JsonKeyExists[F[_]: Applicative](
+  val id: UUID,
+  val key: String
+) extends Predicate[F, Json]:
+
+  /** @inheritDocs
+    */
+  override def eval(input: Json): F[Predicate.Result] =
+    // \\ is recursive, we need a path-based lookup or something
+    Applicative[F].pure(Predicate.Result(input.\\(key).nonEmpty))
+
+object JsonKeyExists:
+
+  def apply[F[_]: Applicative](key: String): JsonKeyExists[F] =
+    new JsonKeyExists[F](UUID.v7(), key)
+
+end JsonKeyExists