Introduction. Notational Conventions Chapter 1. Basic Definitions Chapter 2. The lnvariant Bilinear Form and the Generalized Casimir Operator Chapter 3. Integrable Representations of Kac-Moody Algebras and the Weyl Group Chapter 4. A Classification of Generalized Caftan Matrices Chapter 5. Real and Imaginary Roots Chapter 6. Affine Algebras: the Normalized Invariant Form, the Root System, and the Weyl Group Chapter 7. Affine Algebras as Central Extensions of Loop Algebras Chapter 8. Twisted Affine Algebras and Finite Order Automorphisms Chapter 9. Highest-Weight Modules over Kac-Moody Algebras Chapter 10. Integrable Highest-Weight Modules: the Character Formula Chapter 11. Integrable Highest-Weight Modules: the Weight System and the Unitarizability Chapter 12. Integrable Highest-Weight Modules over Affine Algebras. Application to η-Function Identities. Sugawara Operators and Branching Functions Chapter 13. Affine Algebras, Theta Functions, and Modular Forms Chapter 14. The Principal and Homogeneous Vertex Operator Constructions of the Basic Representation. Boson-Fermion Correspondence. Application to Soliton Equations Index of Notations and Definitions References Conference Proceedings and Collections of Papers