It is now more than five years since the Belgian block cipher Rijndael was chosen as the Advanced Encryption Standard {AES). Joan Daemen and Vincent Rijmcn used algebraic techniques to provide an unparalleled level of assurance against many standard statistical cryptanalytic tech­ niques. The cipher is a fitting tribute to their distinctive approach to cipher design. Since the publication of the AES, however, the very same algebraic structures have been the subject of increasing cryptanalytic attention and this monograph has been written to summarise current research. We hope that this work will be of interest to both cryptogra­ phers and algebraists and will stimulate future research.