text: In response to a request from the United States Congress, OJJDP initiated a project to investigate the methods used by state and local agencies to project future needs for secure capacity. Juvenile justice professionals who must respond to questions about correctional space needs may be tempted to answer with simple statistical predictions based on recent trends in juvenile arrests and court commitments or even recent changes in detention and corrections populations. Simple answers are appreciated because they allow policymakers to proceed with budgeting and construction plans. Repeated experience with estimating future space needs, however, has taught policymakers and practitioners alike that there are no simple answers or, more accurately, that there are no simple and reliable answers. Statistical prediction models are only as good as the data elements that go into them and the assumptions on which they are built. Every juvenile justice administrator eventually learns that the actual demand for detention and corrections space has a way of proving statistical models wrong. Within a few years, policymakers will likely return to ask the same question: How many beds do we need?