diff --git a/packages/context-state/src/create-root.ts b/packages/context-state/src/create-root.ts
new file mode 100644
index 0000000..a4ab750
--- /dev/null
+++ b/packages/context-state/src/create-root.ts
@@ -0,0 +1,35 @@
+import { children } from "./internal"
+import { StateNode } from "./types"
+
+export interface RootNode extends StateNode<never> {
+  run(): () => void
+}
+
+export function createRoot(): RootNode {
+  const childRunners = new Set<(isActive: boolean, value: null) => void>()
+  const runChildren = (isActive: boolean) => {
+    childRunners.forEach((cb) => {
+      cb(isActive, null)
+    })
+  }
+
+  const result: RootNode = {
+    getValue: () => {
+      throw new Error("RootNode doesn't have value")
+    },
+    state$: () => {
+      throw new Error("RootNode doesn't have value")
+    },
+    run: () => {
+      // Maybe more fancy with refcount, etc?
+      runChildren(true)
+      return () => {
+        runChildren(false)
+      }
+    },
+  }
+
+  children.set(result, childRunners)
+
+  return result
+}
diff --git a/packages/context-state/src/internal/children.ts b/packages/context-state/src/internal/children.ts
new file mode 100644
index 0000000..95909de
--- /dev/null
+++ b/packages/context-state/src/internal/children.ts
@@ -0,0 +1,6 @@
+import { StateNode } from "../types"
+
+export const children = new WeakMap<
+  StateNode<any>,
+  Set<(isActive: boolean, value: any) => void>
+>()
diff --git a/packages/context-state/src/internal/index.ts b/packages/context-state/src/internal/index.ts
index 9a38cbf..4c25764 100644
--- a/packages/context-state/src/internal/index.ts
+++ b/packages/context-state/src/internal/index.ts
@@ -1,3 +1,4 @@
 export * from "./empty-value"
 export * from "./errors"
 export * from "./promisses"
+export * from "./children"
diff --git a/packages/context-state/src/substate.test.ts b/packages/context-state/src/substate.test.ts
new file mode 100644
index 0000000..cfaffea
--- /dev/null
+++ b/packages/context-state/src/substate.test.ts
@@ -0,0 +1,224 @@
+import { EMPTY, NEVER, Observable, of, Subject, throwError } from "rxjs"
+import { createRoot } from "./create-root"
+import { substate } from "./substate"
+
+describe("subState", () => {
+  describe("constructor", () => {
+    it("subscribes to the inner observable when the parent has a value", () => {
+      const root = createRoot()
+      const contextSource$ = new Subject<number>()
+      const contextNode = substate(root, () => contextSource$)
+
+      let ranFunction = false
+      substate(contextNode, () => {
+        ranFunction = true
+        return EMPTY
+      })
+
+      expect(ranFunction).toBe(false)
+
+      root.run()
+      expect(ranFunction).toBe(false)
+
+      contextSource$.next(1)
+      expect(ranFunction).toBe(true)
+    })
+
+    it("unsubscribes from the previous observable before subscribing to the new one when the context changes", () => {
+      const root = createRoot()
+      const contextSource$ = new Subject<number>()
+      const contextNode = substate(root, () => contextSource$)
+
+      let subscribed = false,
+        unsubscribed = false
+      substate(contextNode, () => {
+        return new Observable(() => {
+          subscribed = true
+
+          return () => {
+            // the test will reset `subscribed` to false when this function should be run.
+            expect(subscribed).toBe(false)
+            unsubscribed = true
+          }
+        })
+      })
+
+      root.run()
+      contextSource$.next(1)
+      expect(subscribed).toBe(true)
+      expect(unsubscribed).toBe(false)
+
+      subscribed = false
+      contextSource$.next(2)
+      expect(subscribed).toBe(true)
+      expect(unsubscribed).toBe(true)
+    })
+
+    it("can access any observable from the context with the ctx function", () => {
+      const root = createRoot()
+      const contextSource$ = new Subject<number>()
+      const contextNode = substate(root, () => contextSource$)
+
+      let lastContextValue: number | null = null
+      substate(contextNode, (ctx) => {
+        expect(ctx(contextNode)).toBe(lastContextValue)
+        return EMPTY
+      })
+
+      root.run()
+      expect.assertions(3)
+      contextSource$.next((lastContextValue = 1))
+      contextSource$.next((lastContextValue = 2))
+      contextSource$.next((lastContextValue = 3))
+    })
+  })
+
+  describe("getValue", () => {
+    it("throws when the node is not active", () => {
+      const root = createRoot()
+      const subNode = substate(root, () => of(1))
+
+      expect(() => subNode.getValue()).toThrowError("Inactive Context")
+    })
+
+    it("after an error it throws an InactiveContextError", () => {
+      const root = createRoot()
+      const error = new Error()
+      const subNode = substate(root, () => throwError(() => error))
+      root.run()
+
+      expect(() => {
+        console.log(subNode.getValue())
+      }).toThrowError("Inactive Context")
+    })
+
+    it("returns the latest value if the observable has already emitted", () => {
+      const source$ = new Subject<number>()
+      const root = createRoot()
+      const subNode = substate(root, () => source$)
+      root.run()
+
+      source$.next(1)
+      source$.next(2)
+      source$.next(3)
+
+      expect(subNode.getValue()).toBe(3)
+    })
+
+    it("returns a promise that resolves when the first value is emitted", async () => {
+      const source$ = new Subject<number>()
+      const root = createRoot()
+      const subNode = substate(root, () => source$)
+      source$.next(1)
+      root.run()
+
+      const promise = subNode.getValue()
+
+      source$.next(2)
+      source$.next(3)
+
+      await expect(promise).resolves.toBe(2)
+    })
+
+    it("rejects the promise if the node becomes inactive", async () => {
+      const root = createRoot()
+      const subNode = substate(root, () => NEVER)
+      const stop = root.run()
+
+      const promise = subNode.getValue()
+
+      stop()
+
+      await expect(promise).rejects.toBeTruthy()
+    })
+
+    it("rejects the promise when the observable emits an error", async () => {
+      const source$ = new Subject<number>()
+      const root = createRoot()
+      const subNode = substate(root, () => source$)
+      root.run()
+
+      const promise = subNode.getValue()
+
+      const error = new Error()
+      source$.error(error)
+
+      await expect(promise).rejects.toBe(error)
+    })
+
+    it("ignores the observable until its context has a value", async () => {
+      const root = createRoot()
+      const contextSource$ = new Subject<number>()
+      const contextNode = substate(root, () => contextSource$)
+      const source$ = new Subject<number>()
+      const subNode = substate(contextNode, () => source$)
+      root.run()
+
+      source$.next(1)
+
+      const promise = subNode.getValue()
+      expect(promise).toBeInstanceOf(Promise)
+
+      source$.next(2)
+      source$.next(3)
+
+      contextSource$.next(1)
+
+      source$.next(4)
+
+      await expect(promise).resolves.toBe(4)
+    })
+
+    it("discards the old value after a context changes, returning a new promise", async () => {
+      const root = createRoot()
+      const contextSource$ = new Subject<number>()
+      const contextNode = substate(root, () => contextSource$)
+      const source$ = new Subject<number>()
+      const subNode = substate(contextNode, () => source$)
+      root.run()
+
+      contextSource$.next(1)
+      source$.next(2)
+      expect(subNode.getValue()).toBe(2)
+
+      contextSource$.next(3)
+      const promise = subNode.getValue()
+
+      source$.next(4)
+      await expect(promise).resolves.toBe(4)
+    })
+
+    it("returns a promise that yields the value after a context has changed", async () => {
+      const root = createRoot()
+      const contextSource$ = new Subject<number>()
+      const contextNode = substate(root, () => contextSource$)
+      const subNode = substate(contextNode, (ctx) =>
+        ctx(contextNode) === 2 ? of("done!") : EMPTY,
+      )
+      root.run()
+
+      const promise = subNode.getValue()
+
+      contextSource$.next(1)
+      contextSource$.next(2)
+
+      await expect(promise).resolves.toBe("done!")
+    })
+
+    it("rejects the promise if any of its context emits an error", async () => {
+      const source$ = new Subject<number>()
+      const root = createRoot()
+      const subNode = substate(root, () => source$)
+      root.run()
+
+      const promise = subNode.getValue()
+
+      const error = new Error()
+      source$.error(error)
+
+      await expect(promise).rejects.toBe(error)
+    })
+  })
+
+  describe("state$", () => {})
+})
diff --git a/packages/context-state/src/substate.ts b/packages/context-state/src/substate.ts
index f8424da..f6e85fb 100644
--- a/packages/context-state/src/substate.ts
+++ b/packages/context-state/src/substate.ts
@@ -7,13 +7,9 @@ import {
   StatePromise,
   DeferredPromise,
   createDeferredPromise,
+  children,
 } from "./internal"
 
-const children = new WeakMap<
-  StateNode<any>,
-  Set<(isActive: boolean, value: any) => void>
->()
-
 export const ctx = <V>(node: StateNode<V>): V => {
   const value = node.getValue()
   if (value instanceof StatePromise) throw invalidContext()
@@ -24,7 +20,7 @@ export type Ctx = typeof ctx
 export const substate = <T, P>(
   parent: StateNode<P>,
   getState$: (ctx: Ctx) => Observable<T>,
-  equalityFn: (a: T, b: T) => boolean,
+  equalityFn: (a: T, b: T) => boolean = Object.is,
 ): StateNode<T> => {
   let subject: ReplaySubject<T> | null = null
   let subscription: Subscription | null = null