Part I. Elementary Theory: 1. Preliminaries; 2. The constructible universe; 3. ω1-Trees in L; 4. κ+-Trees in L and the fine structure theory; 5. The story of 0#; Part II. Advanced theory: 6. The fine structure theory; 7. Trees and large cardinals in L; 8. Morasses and the cardinal transfer theorem; 9. Silver machines; Remarks and historical notes; Bibliography; Glossary of notation; Index.