diff --git a/src/sim/knowledge.js b/src/sim/knowledge.js
new file mode 100644
index 0000000..272ecbd
--- /dev/null
+++ b/src/sim/knowledge.js
@@ -0,0 +1 @@
+export function markSeen() {}
diff --git a/src/sim/player.js b/src/sim/player.js
new file mode 100644
index 0000000..e044442
--- /dev/null
+++ b/src/sim/player.js
@@ -0,0 +1,6 @@
+export function onPlayerArrive() {}
+export function playerSystem(world) {
+  const loc = world.player.loc;
+  if (loc.kind === 'docked') return world.galaxy.planets[loc.planetId].systemId;
+  return loc.systemId ?? null;
+}
diff --git a/src/sim/world.js b/src/sim/world.js
new file mode 100644
index 0000000..95f457c
--- /dev/null
+++ b/src/sim/world.js
@@ -0,0 +1,91 @@
+import { OBSERVE_INTERVAL, PLAYER_START_CREDITS, PLAYER_CAPACITY } from './constants.js';
+import { generateGalaxy, hopMatrix } from './galaxy.js';
+import { NewsLedger } from './news.js';
+import { EventQueue } from './events.js';
+import { makeRng, hash32 } from './rng.js';
+import { planNpc, onNpcArrive } from './npc.js';
+import { onPlayerArrive } from './player.js';
+import { markSeen } from './knowledge.js';
+
+const NPC_FIRST_NAMES = ['Vask', 'Orla', 'Dex', 'Mira', 'Joss', 'Kade', 'Nyra', 'Tarn', 'Sela', 'Bram'];
+
+export function createWorld(seed, { systemCount = 25, npcCount = 120 } = {}) {
+  const galaxy = generateGalaxy(seed, { systemCount });
+  const rng = makeRng(hash32('world:' + seed));
+  const news = new NewsLedger();
+  news.record(galaxy, 0);
+
+  const npcs = [];
+  for (let i = 0; i < npcCount; i++) {
+    const planet = galaxy.planets[Math.floor(rng() * galaxy.planets.length)];
+    npcs.push({
+      id: i,
+      seed: `w:${seed}:npc:${i}`,
+      name: `${NPC_FIRST_NAMES[i % NPC_FIRST_NAMES.length]}-${i}`,
+      alive: true,
+      credits: Math.floor(1500 + rng() * 2000),
+      capacity: 20 + Math.floor(rng() * 40),
+      cargo: null,
+      planetId: planet.id,
+      systemId: planet.systemId,
+      state: 'docked',
+      plan: null,
+      planCount: 0,
+    });
+  }
+
+  const startPlanet = galaxy.planets[0];
+  const world = {
+    seed, t: 0, galaxy, hops: hopMatrix(galaxy), news, npcs,
+    queue: new EventQueue(),
+    stats: { trades: 0, npcDestroyed: 0 },
+    log: [],
+    player: {
+      credits: PLAYER_START_CREDITS,
+      capacity: PLAYER_CAPACITY,
+      cargo: {},
+      loc: { kind: 'docked', planetId: startPlanet.id },
+      intent: null,
+      tripCount: 0,
+      alive: true,
+      won: false,
+      priceBook: {},
+      seenNpcs: new Set(),
+    },
+  };
+
+  // Stagger initial NPC planning across the first day.
+  for (const npc of npcs) world.queue.push(rng(), 'npc-replan', { npcId: npc.id });
+  world.queue.push(1, 'snapshot', {});
+  world.queue.push(OBSERVE_INTERVAL, 'observe', {});
+  return world;
+}
+
+export function pushLog(world, msg) {
+  world.log.push(`[day ${world.t.toFixed(1)}] ${msg}`);
+  if (world.log.length > 50) world.log.shift();
+}
+
+const HANDLERS = {
+  'npc-replan': (world, data) => planNpc(world, world.npcs[data.npcId]),
+  'npc-arrive': (world, data) => onNpcArrive(world, data),
+  'player-arrive': (world, data) => onPlayerArrive(world, data),
+  snapshot: (world) => {
+    world.news.record(world.galaxy, world.t);
+    world.queue.push(world.t + 1, 'snapshot', {});
+  },
+  observe: (world) => {
+    markSeen(world);
+    world.queue.push(world.t + OBSERVE_INTERVAL, 'observe', {});
+  },
+};
+
+// Advance the world to absolute time toTime, processing all due events in order.
+export function advance(world, toTime) {
+  while (world.queue.size > 0 && world.queue.peek().time <= toTime) {
+    const ev = world.queue.pop();
+    world.t = ev.time;
+    HANDLERS[ev.type](world, ev.data);
+  }
+  world.t = toTime;
+}
diff --git a/tests/world.test.js b/tests/world.test.js
new file mode 100644
index 0000000..f978562
--- /dev/null
+++ b/tests/world.test.js
@@ -0,0 +1,50 @@
+import { describe, it, expect } from 'vitest';
+import { createWorld, advance } from '../src/sim/world.js';
+
+describe('world', () => {
+  it('creates a populated world with player at a planet', () => {
+    const w = createWorld('alpha', { systemCount: 12, npcCount: 30 });
+    expect(w.npcs).toHaveLength(30);
+    expect(w.galaxy.systems).toHaveLength(12);
+    expect(w.player.loc.kind).toBe('docked');
+    expect(w.player.credits).toBeGreaterThan(0);
+    for (const npc of w.npcs) {
+      expect(npc.alive).toBe(true);
+      expect(w.galaxy.planets[npc.planetId]).toBeTruthy();
+    }
+  });
+
+  it('advance processes events; NPCs actually trade', () => {
+    const w = createWorld('alpha', { systemCount: 12, npcCount: 30 });
+    advance(w, 40);
+    expect(w.t).toBe(40);
+    expect(w.stats.trades).toBeGreaterThan(30); // every npc averages >1 completed trade
+  });
+
+  it('is deterministic end-to-end', () => {
+    const run = () => {
+      const w = createWorld('alpha', { systemCount: 12, npcCount: 30 });
+      advance(w, 40);
+      return w.npcs.map((n) => [n.credits, n.planetId, n.alive]);
+    };
+    expect(JSON.stringify(run())).toBe(JSON.stringify(run()));
+  });
+
+  it('different seeds diverge', () => {
+    const w1 = createWorld('alpha', { systemCount: 12, npcCount: 30 });
+    const w2 = createWorld('beta', { systemCount: 12, npcCount: 30 });
+    advance(w1, 20);
+    advance(w2, 20);
+    expect(JSON.stringify(w1.npcs.map((n) => n.credits)))
+      .not.toBe(JSON.stringify(w2.npcs.map((n) => n.credits)));
+  });
+
+  it('advance is incremental: many small steps equal one big step', () => {
+    const a = createWorld('alpha', { systemCount: 12, npcCount: 30 });
+    const b = createWorld('alpha', { systemCount: 12, npcCount: 30 });
+    advance(a, 20);
+    for (let t = 0.5; t <= 20.0001; t += 0.5) advance(b, t);
+    expect(JSON.stringify(b.npcs.map((n) => [n.credits, n.planetId])))
+      .toBe(JSON.stringify(a.npcs.map((n) => [n.credits, n.planetId])));
+  });
+});