Using examples from evaluations of the Emergency School Aid Act (ESAA) Basic Grants Program, the ESAA Pilot Program, and the sustaining effects of compensatory education programs, school and student attrition are discussed. As in the example cases, appreciable attrition can be expected in most longitudinal studies. The possible effects of this attrition on descriptive analyses, analyses of student gains for each school year, and analyses of differential achievement gains for different treatment groups are considered. The decision of whether to use statistical adjustments for bias is also analyzed. (BW)