It is a special characteristic of all modern societies that people consciously decide on and plan projects designed to improve their social systems. It is their universal predicament that their projects do not always have their intended effects. It seems inevitable that in most countries this common set of problems, combined with the obvious relevance of social science research procedures, will have generated a methodology and methodological specialists focused on the problem of assessing the impact of planned social change. It is an assumption of this paper that, in spite of differences in forms of government and approaches to social planning and problem-solving, much of this methodology can be usefully shared--that social project evaluation methodology is one of the fields of science that has enough universality to make scientific sharing mutually beneficial. As a part of this sharing, the author reports on program impact assessment methodology as it is developing in the United States today. He focuses on some problems encountered in the U.S. experience with assessing the impact of planned social change. He has grouped the problems under three general headings. First comes issues that are internal to scientific community and would be present even if scientist program evaluators were running society solely for the purpose of unambiguous program evaluation. These are: (1) Metascientific issues; and (2) Statistical issues. The remaining heading involves interaction with the societal context. Under (3) Political system problems, the author deals with issues that specifically involve political processes and governmental institutions, some of which are perhaps common to all large, bureaucratic nations, other unique to the U.S. setting. (Contains 9 figures.)