In this book, awardwinning author Goro Shimura treats new areas and presents relevant expository material in a clear and readable style. Topics include Witt's theorem and the Hasse principle on quadratic forms, algebraic theory of Clifford algebras, spin groups, and spin representations. He also includes some basic results not readily found elsewhere. The two principle themes are: (1) Quadratic Diophantine equations; (2) Euler products and Eisenstein series on orthogonal groups and Clifford groups. The starting point of the first theme is the result of Gauss that the number of primitive representations of an integer as the sum of three squares is essentially the class number of primitive binary quadratic forms. Presented are a generalization of this fact for arbitrary quadratic forms over algebraic number fields and various applications. For the second theme, the author proves the existence of the meromorphic continuation of a Euler product associated with a Hecke eigenform on a Clifford or an orthogonal group. The same is done for an Eisenstein series on such a group. Beyond familiarity with algebraic number theory, the book is mostly selfcontained. Several standard facts are stated with references for detailed proofs. Goro Shimura won the 1996 Steele Prize for Lifetime Achievement for "his important and extensive work on arithmetical geometry and automorphic forms". Readership Graduate students and research mathematicians interested in number theory and algebraic groups. Table of Contents  Introduction
 Algebraic theory of quadratic forms, Clifford algebras, and spin groups
 Quadratic forms, Clifford groups, and spin groups over a local or global field
 Quadratic diophantine equations
 Groups and symmetric spaces over R
 Euler products and Eisenstein series on orthogonal groups
 Euler products and Eisenstein series on Clifford groups
 Appendix
 References
 Frequently used symbols
 Index
