This book provides a comprehensive survey of different kinds of Feistel ciphers, including their definition and mathematical/computational properties. Feistel Networks form the base design of the Data Encryption Standard algorithm, a former US NIST standard block cipher, originally released in 1977, and the framework used by several other symmetric ciphers ever since. The results consolidated in this volume provide an overview of this important cipher design to researchers and practitioners willing to understand the design and security analysis of Feistel ciphers.