Since the early 1980s we have been engaged in the design, implementation, and evaluation of "cognitive acceleration" programs, which claim to enhance students' general intellectual processing skills. We have published extensive data showing long-term effects of these programs on students? cognitive development and academic achievement. In this paper the theory base of cognitive acceleration will be briefly outlined and some typical results re-presented. The main body of the paper will be devoted to a discussion of a number of issues which arise from the analysis of long-term effects. Although the examples will be drawn from the cognitive acceleration programs, the issues are general ones which might apply to any long-term study.