A larger proportion than ever before of American women (including minority women and disabled women) will have to become available to join the S/E workforce during the next two decades, if only to help stem the decline in the total S/E workforce now expected on the basis of demographic data. An important transition phase in the careers of future women scientists/engineers is the immediate postdoctoral period, during which a serious differential in participation in S/E occurs. Previous attempts to understand such attritions have been only partly successful. We therefore shall undertake a major effort (building on our experience with a pilot project) to study the whole population of women scientists/ engineers, as well as matched samples of men as control, all of whom in the past received Postdoctoral Fellowships from NSF and NRC. Quantitative and qualitative data, obtained through standard survey and analysis methods, will be used to provide a fuller description of the existing barriers, and to formulate and test propositions that can help guide policy for their amelioration or removal.