The Survey of Program Dynamics (SPD) was created by the 1996 welfare reform legislation to facilitate its evaluation. This paper describes the evolution of that survey, discusses its implementation, and draws lessons for future evaluation. Large-scale surveys can be an important part of a portfolio of evaluation methods, but sufficient time must be given to data collection agencies if a high-quality longitudinal survey is expected. Such a survey must have both internal (agency) and external (policy analyst) buy-in. Investments in data analysis by agency staff, downplayed in favor of larger sample sizes given a fixed budget, could have contributed to more external acceptance. More attention up-front to reducing the potentially deleterious effects of attrition in longitudinal surveys, such as through the use of monetary incentives, might have been worthwhile. Given the problems encountered by the Census Bureau in producing the SPD, I argue that ongoing multi-purpose longitudinal surveys like the Survey of Income and Program Participation are potentially more valuable than episodic special-purpose surveys.