For decades, school finance reform in many states has been driven by (or reacted to) litigation arguing that schools need additional funding. In the 1970s and 80s, school finance cases prompted reforms to increase equity, equalizing dollars per pupil across districts. In the 1990s, the focus began shifting to educational outcomes, with a recognition that equal dollars do not always buy equal outcomes because some districts face higher costs than others. Thus, the courts, and resulting policies, have asked what dollar amount is sufficient, or "adequate", in any given district, to provide the level of educational quality that the state requires. This report provides an overview of the large literature that has developed to measure "adequacy", or the per-pupil cost of providing K-12 education in districts with varying characteristics, and the implications for the design of school finance systems. In general, the cost of an "adequate education" can be defined as the minimum amount of money that a school district must spend in order to achieve a targeted educational outcome, such as students achieving a particular score on state tests. Section I of this paper briefly reviews the methods used to measure the costs of adequacy. Section II discusses specific factors that may impact costs, with associated estimates of the size of marginal impacts, to the extent that there is research available, or notes when the literature is inconclusive. Section II also highlights how other states, particularly those most similar to California, address various cost factors in their funding systems. Section III moves to discussion of how states might structure their funding systems in order to pay for the costs of adequacy, again with a focus on the systems in states most similar to California. Section IV summarizes the implications for California.