One of the first of a new generation of books in mathematics that show the reader how to do large or complex computations using the power of computer algebra. It contains descriptions of 148 algorithms, which are fundamental for number theoretic calculations, in particular for computations related to algebraic number theory, elliptic curves, primality testing, lattices and factoring. For each subject there is a complete theoretical introduction. A detailed description of each algorithm is given allowing for immediate computer implementation. Many of the algorithms are new or appear for the first time in this book. A large number of exercises is also included.