Introgression is the incorporation of a gene from one organism complex into another as a result of hybridization. A major concern of the use of genetically modified plants is the unintentional spread of the new genes from cultivated plants to their wild relatives and the subsequent impacts on the ecology of wild plants and their associated flora and fauna. This book reviews these issues, focusing on the ecological and evolutionary effects of introducing GM cultivars. It presents current knowledge of crop-wild relatives hybridization and introgression, and the measurement and prediction of their consequences. As a result it represents a major contribution to the debate about the risks of GM crops and measures, such as post commercialisation monitoring, required to determine the longer term impacts of GM crops on ecosystems.