This book is a revised and updated version, including a substantial portion of new material, of the authors' widely acclaimed earlier text, Perturbation Methods in Applied Mathematics (AMS 34). A new chapter dealing with regular expansions has been added, the discussion of layer-type singular perturbations has been revised, and the coverage of multiple scale and averaging methods has been significantly expanded to reflect recent advances and viewpoints. The result is a comprehensive account of the various perturbation techniques currently used in the sciences and engineering that is suitable both as graduate text and as a reference work on the subject.