Sperm-mediated gene transfer (SMGT) represents a novel set of technologies for animal (or in the future, human) genetic modification using the sperm as a vector, as opposed to more traditional established routes such as fertilized eggs or embryonic stem cells. Studies of sperm-mediated gene transfer (SMGT) indicate that sperm cells possess the ability to be utilized as carriers of exogenous genetic sequences, offering the potential of a novel cost-effective route for germline genetic modification. The fate of transgenes borne by sperm cells has been inconsistent, and analysis of offspring from SMGT experiments has shown a mixed picture in terms of genomic integration of the transgene, suggesting an episomal mode of inheritance. Various distinct steps in transgene uptake by the sperm cell have been described or proposed, including a model based upon endogenous reverse transcriptase activity. Although mature sperm cells are naturally protected against uptake of foreign nucleic acid molecules, certain environmental conditions, for example at key times within the reproductive tract, may reduce this protection, suggesting that SMGT may occasionally take place in nature. If correct, this carries profound implications for evolution and human genetic health. This book brings together theoretical and empirical reviews from experts in SMGT, providing comprehensive coverage of the major trends, developments and controversies in this novel field. This book is intended as a reference for professional researchers in the field of animal genetic modification (transgenesis) as well as teachers, scientists and physicians interested in medical genetics in general and gene therapy in particular.