diff --git a/src/ledger/user.js b/src/ledger/user.js
new file mode 100644
index 0000000..fd24676
--- /dev/null
+++ b/src/ledger/user.js
@@ -0,0 +1,105 @@
+// @flow
+
+/**
+ * This module has a core data type identifying SourceCred users.
+ *
+ * The scope for this data type is to model:
+ * - a unique identifier for each user
+ * - a unique (renameable) username they choose
+ * - the address of every node they correspond to in the graph
+ *
+ * Unlike most other state in SourceCred, the User state is
+ * nondeterministically generated by SourceCred itself, and then persisted
+ * long-term within the instance.
+ *
+ * This is in contrast to Graph data, which usually comes from an external
+ * source, and is not persisted long-term, but instead is re-generated when
+ * needed.
+ *
+ * In particular, this kernel of user data is stored within the core ledger,
+ * since it's necessary to track consistently when tracking Grain distribution.
+ * This type should not grow to include all the data that the UI will
+ * eventually want to show; that should be kept in a different data store which
+ * isn't being used as a transaction ledger.
+ *
+ */
+import {
+  type Uuid,
+  parser as uuidParser,
+  random as randomUuid,
+} from "../util/uuid";
+import * as C from "../util/combo";
+import {
+  type NodeAddressT,
+  NodeAddress,
+  type Node as GraphNode,
+} from "../core/graph";
+
+/**
+ * We validate usernames using GitHub-esque rules.
+ */
+export opaque type Username: string = string;
+const USERNAME_PATTERN = /^@?([A-Za-z0-9-_]+)$/;
+
+export type UserId = Uuid;
+export type User = {|
+  // UUID, assigned when the user is created.
+  +id: UserId,
+  +name: Username,
+  // Every other node in the graph that this user corresponds to.
+  // Does not include the user's "own" address, i.e. the result
+  // of calling (userAddress(user.id)).
+  +aliases: $ReadOnlyArray<NodeAddressT>,
+|};
+
+// It's not in the typical [owner, name] format because it isn't provided by a plugin.
+// Instead, it's a raw type owned by SourceCred project.
+export const USER_PREFIX = NodeAddress.fromParts(["sourcecred", "USER"]);
+
+/**
+ * Create a new user, assigning a random id.
+ */
+export function createUser(name: string): User {
+  return {
+    id: randomUuid(),
+    name,
+    aliases: [],
+  };
+}
+
+/**
+ * Parse a Username from a string.
+ *
+ * Throws an error if the username is invalid.
+ */
+export function usernameFromString(username: string): Username {
+  const re = new RegExp(USERNAME_PATTERN);
+  const match = re.exec(username);
+  if (match == null) {
+    throw new Error(`invalid username: ${username}`);
+  }
+  return match[1];
+}
+
+export function userAddress(id: UserId): NodeAddressT {
+  return NodeAddress.append(USER_PREFIX, id);
+}
+
+export function graphNode({id, name}: User): GraphNode {
+  return {
+    address: userAddress(id),
+    description: name,
+    timestampMs: null,
+  };
+}
+
+export const usernameParser: C.Parser<Username> = C.fmap(
+  C.string,
+  usernameFromString
+);
+
+export const userParser: C.Parser<User> = C.object({
+  id: uuidParser,
+  name: usernameParser,
+  aliases: C.array(C.fmap(C.string, NodeAddress.fromRaw)),
+});
diff --git a/src/ledger/user.test.js b/src/ledger/user.test.js
new file mode 100644
index 0000000..d5eeae2
--- /dev/null
+++ b/src/ledger/user.test.js
@@ -0,0 +1,53 @@
+// @flow
+
+import deepFreeze from "deep-freeze";
+import {fromString as uuidFromString} from "../util/uuid";
+import {NodeAddress} from "../core/graph";
+import {
+  createUser,
+  userAddress,
+  usernameFromString,
+  USER_PREFIX,
+  graphNode,
+  type User,
+} from "./user";
+
+describe("ledger/user", () => {
+  const uuid = uuidFromString("YVZhbGlkVXVpZEF0TGFzdA");
+  const name = usernameFromString("foo");
+  const example: User = deepFreeze({
+    id: uuid,
+    name,
+    aliases: [NodeAddress.empty],
+  });
+  it("createUser works", () => {
+    const user = createUser(name);
+    expect(user.aliases).toEqual([]);
+    expect(user.name).toEqual(name);
+    // Verify it is a valid UUID
+    uuidFromString(user.id);
+  });
+  it("userAddress works", () => {
+    expect(userAddress(uuid)).toEqual(NodeAddress.append(USER_PREFIX, uuid));
+  });
+  it("graphNode works", () => {
+    const node = graphNode(example);
+    expect(node.description).toEqual(example.name);
+    expect(node.address).toEqual(userAddress(uuid));
+    expect(node.timestampMs).toEqual(null);
+  });
+  describe("usernameFromString", () => {
+    it("fails on invalid usernames", () => {
+      const bad = ["With Space", "With.Period", "A/Slash", ""];
+      for (const b of bad) {
+        expect(() => usernameFromString(b)).toThrowError("invalid username");
+      }
+    });
+    it("succeeds on valid usernames", () => {
+      const names = ["h", "hi_there", "ZaX99324cab"];
+      for (const n of names) {
+        expect(usernameFromString(n)).toEqual(n);
+      }
+    });
+  });
+});