This book is a unique compilation of studies and research by the author during over 20 years, on the development of insect pest biotypes that have adapted to and can damage varieties that have been incorporated resistance to them. They appear because of the selection pressure exerted by these varieties on insects when they are cultivated in extended areas. These subpopulations, really called insect xenotypes, have developed in important food crops, including small grains, sorghum, corn, rice, alfalfa and other legume crops, cabbage, raspberry, wine grapes, etc., in many countries. The development of resistant varieties, however, will continue offering an alternative to traditional pest management strategies.