1. Introduction; 2. Early research on fission, and overview: 1933-43; 3. The early materials program: 1933-43; 4. Setting up project Y: June 1942 to March 1943; 5. Research in the first months of project Y: April to September 1943; 6. Creating a wartime community: September 1943 to August 1944; 7. The gun weapon: September 1943 to August 1944; 8. The implosion program accelerates: September 1943 to July 1944; 9. New hopes for the implosion weapon: September 1943 to July 1944; 10. The nuclear properties of a fission weapon: September 1943 to July 1944; 11. Uranium and plutonium: early 1943 to August 1944; 12. The discovery of spontaneous fission in plutonium and the reorganisation of Los Alamos; 13. Building the uranium bomb: August 1944 to July 1945; 14. Exploring the plutonium implosion weapon: August 1944 to February 1945; 15. Finding the implosion design: August 1944 to February 1945; 16. Building the implosion gadget: March 1945 to July 1945; 17. Critical assemblies and nuclear physics: August 1944 to July 1945; 18. The test at Trinity: January 1944 to July 1945; 19. Delivery: June 1943 to August 1945; 20. The legacy of Los Alamos.