diff --git a/contracts/Marketplace.sol b/contracts/Marketplace.sol
new file mode 100644
index 0000000..3629151
--- /dev/null
+++ b/contracts/Marketplace.sol
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+
+contract Marketplace {
+  IERC20 public immutable token;
+  Totals private totals;
+  mapping(bytes32 => Request) private requests;
+
+  constructor(IERC20 _token) invariant {
+    token = _token;
+  }
+
+  function transferFrom(address sender, uint256 amount) private {
+    address receiver = address(this);
+    require(token.transferFrom(sender, receiver, amount), "Transfer failed");
+  }
+
+  function requestStorage(Request calldata request) public invariant {
+    bytes32 id = keccak256(abi.encode(request));
+    require(request.size > 0, "Invalid size");
+    require(requests[id].size == 0, "Request already exists");
+    requests[id] = request;
+    transferFrom(msg.sender, request.maxPrice);
+    totals.received += request.maxPrice;
+    totals.balance += request.maxPrice;
+    emit StorageRequested(id, request);
+  }
+
+  struct Request {
+    uint256 duration;
+    uint256 size;
+    bytes32 contentHash;
+    uint256 proofPeriod;
+    uint256 proofTimeout;
+    uint256 maxPrice;
+    bytes32 nonce;
+  }
+
+  event StorageRequested(bytes32 id, Request request);
+
+  modifier invariant() {
+    Totals memory oldTotals = totals;
+    _;
+    assert(totals.received >= oldTotals.received);
+    assert(totals.sent >= oldTotals.sent);
+    assert(totals.received == totals.balance + totals.sent);
+  }
+
+  struct Totals {
+    uint256 balance;
+    uint256 received;
+    uint256 sent;
+  }
+}
diff --git a/test/Marketplace.test.js b/test/Marketplace.test.js
new file mode 100644
index 0000000..174c1ec
--- /dev/null
+++ b/test/Marketplace.test.js
@@ -0,0 +1,83 @@
+const { ethers } = require("hardhat")
+const { expect } = require("chai")
+const { exampleRequest } = require("./examples")
+const { keccak256, defaultAbiCoder } = ethers.utils
+
+describe("Marketplace", function () {
+  const request = exampleRequest()
+
+  let marketplace
+  let token
+  let accounts
+
+  beforeEach(async function () {
+    const TestToken = await ethers.getContractFactory("TestToken")
+    token = await TestToken.deploy()
+    const Marketplace = await ethers.getContractFactory("Marketplace")
+    marketplace = await Marketplace.deploy(token.address)
+    accounts = await ethers.getSigners()
+    await token.mint(accounts[0].address, 1000)
+  })
+
+  describe("requesting storage", function () {
+    it("emits event when storage is requested", async function () {
+      await token.approve(marketplace.address, request.maxPrice)
+      await expect(marketplace.requestStorage(request))
+        .to.emit(marketplace, "StorageRequested")
+        .withArgs(requestId(request), requestToArray(request))
+    })
+
+    it("rejects request with insufficient payment", async function () {
+      let insufficient = request.maxPrice - 1
+      await token.approve(marketplace.address, insufficient)
+      await expect(marketplace.requestStorage(request)).to.be.revertedWith(
+        "ERC20: transfer amount exceeds allowance"
+      )
+    })
+
+    it("rejects requests of size 0", async function () {
+      let invalid = { ...request, size: 0 }
+      await token.approve(marketplace.address, invalid.maxPrice)
+      await expect(marketplace.requestStorage(invalid)).to.be.revertedWith(
+        "Invalid size"
+      )
+    })
+
+    it("rejects resubmission of request", async function () {
+      await token.approve(marketplace.address, request.maxPrice * 2)
+      await marketplace.requestStorage(request)
+      await expect(marketplace.requestStorage(request)).to.be.revertedWith(
+        "Request already exists"
+      )
+    })
+  })
+})
+
+function requestId(request) {
+  return keccak256(
+    defaultAbiCoder.encode(
+      [
+        "uint256",
+        "uint256",
+        "bytes32",
+        "uint256",
+        "uint256",
+        "uint256",
+        "bytes32",
+      ],
+      requestToArray(request)
+    )
+  )
+}
+
+function requestToArray(request) {
+  return [
+    request.duration,
+    request.size,
+    request.contentHash,
+    request.proofPeriod,
+    request.proofTimeout,
+    request.maxPrice,
+    request.nonce,
+  ]
+}
diff --git a/test/examples.js b/test/examples.js
index 9ec4da6..05452c4 100644
--- a/test/examples.js
+++ b/test/examples.js
@@ -1,13 +1,15 @@
 const { ethers } = require("hardhat")
 const { now, hours } = require("./time")
+const { sha256, hexlify, randomBytes } = ethers.utils
 
 const exampleRequest = () => ({
   duration: 150, // 150 blocks ≈ half an hour
   size: 1 * 1024 * 1024 * 1024, // 1 Gigabyte
-  contentHash: ethers.utils.sha256("0xdeadbeef"),
+  contentHash: sha256("0xdeadbeef"),
   proofPeriod: 8, // 8 blocks ≈ 2 minutes
   proofTimeout: 4, // 4 blocks ≈ 1 minute
-  nonce: ethers.utils.randomBytes(32),
+  maxPrice: 42,
+  nonce: hexlify(randomBytes(32)),
 })
 
 const exampleBid = () => ({
@@ -16,7 +18,7 @@ const exampleBid = () => ({
 })
 
 const exampleLock = () => ({
-  id: ethers.utils.randomBytes(32),
+  id: hexlify(randomBytes(32)),
   expiry: now() + hours(1),
 })