This Research and Development (R&D) report describes the process of generating comparable estimates of undergraduate enrollment in postsecondary institutions across four National Center for Education Statistics (NCES) datasets--the National Postsecondary Student Aid Study (NPSAS), a sample survey of postsecondary students; the Integrated Postsecondary Education Data System (IPEDS), a universe survey of postsecondary institutions; the National Household Education Surveys Program (NHES) Adult Education Survey, a sample survey of adults in households; and the October school enrollment supplement to the Current Population Survey (CPS), a sample survey of adults in households. The purpose of the report is to highlight differences across surveys that may affect postsecondary enrollment estimates and to describe how largely comparable estimates can be derived, given these differences. For each dataset, the analysis estimated the number of individuals enrolled in postsecondary education in the 1989-90, 1995-96, and 1999-2000 school years, or the closest available time period to those dates. Enrollment counts were estimated for the traditional college age group, ages 18 to 24, as well as for those individuals ages 18 to 64. Each estimate was placed over the relevant population age group to obtain an estimate of the percentage of the population enrolled in postsecondary education, using resident population counts (for April 1990, 1996, and 2000) provided by the U.S. Census Bureau. A section on data sources and limitations is appended. (Contains 8 tables and 7 figures.)