Pressing questions about the merits of full-fledged market accountability in K-12 education, and more limited choice programs, have spawned a large scholarly literature. This article assesses what we know from the most prominent studies and the importance of those findings to school system reform discussions. The studies most widely cited in the United States based their findings on small-scale, restriction-laden U.S. programs, and a handful of larger, but still restriction-laden, foreign school choice programs. The extensive restrictions and small size of those programs make them virtually irrelevant to the widely acknowledged transformational change imperative. Because the most intensely studied programs lack most, and sometimes all, of the key elements of market systems, such as profit, price change, market entry, and product differentiation, they don't help us determine whether unleashing market forces will produce more desired than undesired effects. However, absence of the key elements of market systems did not stop numerous analysts from stating conclusions about "markets," and "competition." So we know a lot about some of the policy approaches to increased school choice, but little about many policy approaches, and less than many people think we know. We know the most about programs and policies too limited in scope to yield much insight into reform policies capable of addressing the transformation imperative implied by the repeatedly alleged "nation at risk" problem. (Contains 203 notes.)