This volume reports on the productivity impacts of varietal improvement research conducted at a number of international centers affiliated with the Consultative Group on International Agricultural Research. Such centers have been at the forefront of a 40-year effort to breed new varieties of the world's staple food crops. The volume is the main product of a study initiated and supported by the Impact Assessment and Evaluation Group (now Standing Panel on Impact Assessment) of the CGIAR. Descriptive data and econometric models are used to evaluate the impact of research on eleven crops and in three country case studies (Brazil, China and India).