The chromosomes of living organisms were once thought to be very stable with changes in genetic content infrequent and generally small. We now know all chromosomes contain a multitude of mobile genetic elements that can change position, cause mutation, and proliferate. This book describes how such elements can influence the biology and evolution of all organisms and provides important unifying information on mechanisms of element mobility. Moreover, the book contains invaluable methodologies for using mobile genetic elements as tools in the analysis of gene structure and function.