How do students learn? What facilitates or impedes learning? Of the myriad potentially important factors, which ones are the most important? These questions, at once deceptively simple and perilously complex, lie at the heart of much educational research. A universal answer has eluded scholars and philosophers for centuries and remains, for some, the holy grail of educational research today. Does the persistence of these questions suggest that we, as a community of scholars, have learned nothing in our decades of study? Or is the inquiry into student learning a perpetual search destined never to end? The importance of questions about student learning requires that we carefully consider, and as carefully evaluate, any serious attempt to address them. From this perspective, I applaud Margaret Wang, Geneva Haertel, and Herbert Walberg (1993a) for tackling difficult questions. Taken at face value, their goals-to identify and estimate the influence of educational, psychological, and social factors on learning ... [so that they may] quantify the importance and consistency of variables that influence learning-seem laudable. If these goals could be met and incontrovertible answers found, the information would be useful to many. Federal, state, and local policymakers would be better able to calibrate priorities and implement intervention strategies. School administrators and classroom teachers would be better able to develop curricula and deliver educational services. And the educational research community would be able to cite a discovery comparable to a treatment for cancer or a marker gene for Huntington's disease. Are these goals attainable? I agree with Wang, Haertel, and Walberg (1993a) that no individual study, regardless of size or design, could answer these questions once and for all. Even a large number of well-designed field experiments could not yield a universal answer for all groups of learners, in all contexts, for all times. So the authors turn to the principle of meta-analysis, one of the most significant methodological advances of the past two decades. Meta-analysis, in all its many forms, has quickly become an extraordinarily valuable component of our research repertoire; it has changed both the content, and our perceptions, of virtually every review article we read. Its principles and structure have trans-