Transfer of alien genes into crop plants from wild and distant plant genetic resources has invoked tremendous interest of crop scientists globally and several traits including resistance to diseases and insect-pests, tolerance to drought, salinity, temperature extremities and other abiotic stresses as well as genes for several quality traits have been transferred through vertical and horizontal gene transfer. Alien gene transfer, lately aided by molecular markers,  molecular cytogenetics, genetic transformation and improved in vitro techniques has led to introgression of hundreds of genes of interest in crop species, thereby widening their genetic base and improving their genetic potential. While the gains through alien transfer are tremendous, these have also raised some doubts about the long-term economic and ecological impacts of such transfers in cultivated background. This book addresses all these issues and provides an insight into the methods, newer innovations, detection and achievements of alien gene transfer in crop plants. At the same time it also focuses on the issues of possible human and ecological impacts of alien gene transfers and describes the challenges and risks involved.