Revised version of a paper presented to the Workshop on "The Future of Impact Assessment in CGIAR: Needs, Constraints, and Options", Standing Panel on Impact Assessment (SPIA) of the Technical Advisory Committee, Rome, May 3-6, 2000, FAO, Rome. This paper summarizes "state of the art" in ex post economic impact assessment of agricultural research with an emphasis on providing a practical guide that can be used by research managers and economists working within a research organization under tight time and resource constraints. Much of the early part of the paper is focused on conceptual and methodological issues. Later sections of the paper turn to implementation issues, especially the implementation of impact assessment as a routine activity. Throughout, the focus is on public research organizations in developing countries, both national and international, and only in one section do we focus on the emphasize the special issues in international research organizations. The underlying premise is that evaluation work in the CGIAR should increasingly be based on evaluation work undertaken in NARSs, with IARCs providing a facilitating and synthetic role.