Investments in agricultural research by national and international organizations have successfully generated improvements in the economic well-being of people, well quantified in a wide range of ex-post impact assessment studies. In contrast, relatively little attention has been given to quantifying the impacts of research on the environment. This gap in understanding the full range of impacts arising from agricultural research presents an important challenge. Growing scientific and public recognition of the significance of environmental impacts of agricultural research, both positive and negative, necessitates their integration into the research evaluation process. This article provides a broad overview of some of the conceptual issues and empirical challenges inherent in measuring and documenting environmental impacts resulting from changes in agricultural practices, reviews some recent environmental impact assessment case studies, and discusses the lessons yielded by those case studies.