The existence of transposable elements was implied by Barbara McClintock in the 1940s, based on genetic experiments, and verified in the 1970s by molecular biology. By the late 1990s, hundreds of elements have been detected in all living organisms and are believed to represent more than 15% of every organism's genome. Major questions remain to be answered concerning how transposable elements have evolved within the context of interacting host genomes. These questions are of great interest because transposable elements may have had a significant impact on the evolution of host genome structure and the ability of populations and species to successfully adapt to their environments. In this book, complementary aspects of the evolution impact of transposable elements are discussed in papers presented by participants of the ESF workshop entitled Evolution and Role of Transposable Elements which was held at CNRS in Gif-sur-Yvette, France in September 1996.