Spermiogenesis summarizes the process of sperm structure and development in a variety of organisms. It attempts to illustrate the structure of the organelles that characterize mature spermatozoa and to trace the morphogenesis of these organelles during sperm formation. This task is complicated by considerable variations in the structure of spermatozoa among different species and the lack of good morphological studies of sperm structure and spermiogenesis in many animal groups. The general morphological features of mammalian sperm structure differ in some ways from the features of insect sperm, the spermatozoa produced by insects appearing relatively more simple. When the functions and developmental regions of the organelles are considered, homologies between complex structures found in mammalian sperm and simpler insect sperm structures may be deduced even though the organelles in question are quite dissimilar in appearance. Such homologies will be stressed in an effort to present as general a picture of spermiogenesis as seems consistent with the species-specific variations that exist. This book will be a useful, stimulating reference for students of reproductive biology.