During the past century wheat breeders have produced a large number of genetically improved wheat lines and varieties. This activity has led to widespread adoption of improved varieties, a steady increase in average wheat yields during the past 4–5 decades and major contributions to food security and poverty reduction. The rate of generation and adoption of improved varieties, and therefore the time lag from varietal release to widespread use, varies across regions. The remarkable success of wheat improvement hinges on the decisions of millions of farmers to adopt, or replace older wheat varieties with superior material. The present paper summarizes studies on the determinants of adoption. Because adoption is a necessary but not sufficient condition for economic impact, the present paper synthesizes key assessments of impact from different farming systems in developing countries.