This paper reports on an assessment of the impact of maize research in Sub- Saharan Africa (SSA), based on case studies of Senegal, Nigeria, Zaire, Kenya and Malawi. Its principal finding is that maize production, much of it on smallscale farms, has virtually doubled in the last 30 years, rising at on average over 2.5% per annum, of which 1.8% is accounted for by area expansion and 0.7% by yield increases. It argues that without research, the negative influence of worsening pest regimes and decreasing fertility would have implied declining, not constant yields for maize. Part of the impact of research is therefore obscured. It is further obscured by the fact that numerous impacts important to African fanners other than increases in maize output have taken place: increases in per hectare productivity have, for instance, allowed higher returns to labour (in many areas a crucially limiting factor) to be obtained, sometimes accompanied by reductions in maize area or switching of maize production among different land types. Important reductions in risk have also occurred where eg early maturing varieties have reduced the "hungry period", or where yields have become more stable in the face of unfavourable weather or pest problems. In some cases, maize has provided a stable source of capital which farmers have accumulated to invest in more profitable non-farm activities such as trade. These findings, again part of the observed impact of technological change, are difficult to quantify, but have important methodological implications for future research impact assessments.