1988 marked the first centenary of Recursion Theory, since Dedekind''s 1888 paper on the nature of number. Now available in paperback, this book is both a comprehensive reference for the subject and a textbook starting from first principles.Among the subjects covered are: various equivalent approaches to effective computability and their relations with computers and programming languages; a discussion of Church''s thesis; a modern solution to Post''s problem; global properties of Turing degrees; and a complete algebraic characterization of many-one degrees. Included are a number of applications to logic (in particular Gödel''s theorems) and to computer science, for which Recursion Theory provides the theoretical foundation.