This is a survey, accessible to advanced undergraduates, of some parts of abstract algebra that are of use in many other areas of discrete mathematics. The treatment is mathematical, but the authors have made great efforts to address the needs of those who will be using the techniques that are being discussed. The presentation assumes knowledge of the material covered in a course on linear algebra and some acquaintance with the basics of groups, rings, and fields. More than 500 exercises accompany the text; fully worked out computational examples are especially emphasized. This new edition has been substantially revised. It includes corrections and improvements to the first four chapters; an enlarged chapter on applications of groups; and two new chapters, one on cryptology and the other an extensive survey of mostly recent applications, many of which are not commonly found in undergraduate texts. A complete solutions manual is available.