Few questions in public education discourse benefit as much from research-based evidence as the question of class size--the pursuit of the ideal number of students that should be co-located for any particular period of instruction. But for policymakers, research on class size can be an embarrassment of riches, and much of the research appears to conflict--literature reviews often find that the number of studies boasting significant returns on investment offered by class size reduction (CSR) equals the number citing its ineffectiveness. Still more reports suggest no significant effects at all. In this "Chief's Pocket Guide," the authors draw from the major studies and literature reviews of CSR to find that, when viewed in isolation--a practice not recommended--dramatic reductions in class size are associated with desirable outcomes, including higher levels of student learning, and the typically modest effects are more pronounced in the early grades and for low-income minority students. When viewed from a whole-systems perspective, however, CSR efforts require a comprehensive consideration of resources, and therefore may not be the most cost-effective approach to improving student learning. This "Chiefs' Pocket Guide" will examine: (1) A brief history of high-profile CSR efforts; (2) A synthesis of research on the effects of CSR efforts; (3) Policy considerations for state education chiefs who may be implementing, considering, or reconsidering CSR as part of their reform agenda; and (4) Arguments for designing systems that move beyond the "class size" construct. A bibliography is included. (Contains 2 figures.)