American philanthropy, by local and national foundations, corporations, and wealthy individuals, has played many important roles in K-12 education: creating new schools, underwriting research, funding scholarships, testing hypotheses, generating new curricula, invoking ideals, setting agendas, bolstering training, and building a case for policy changes. This article discusses some of the resulting programs, with a particular focus on programs dealing with teacher improvement. One major problem with such philanthropy is that it is often difficult to tell whether a foundation is making a difference. Outside of evaluations paid for by the foundations themselves or even done internally, philanthropy often receives little scrutiny, and philanthropists are often treated like celebrities. However, it is probably easier now than in the past to determine the impact of philanthropy through required state-wide tests. But some worry that demanding rapid test-score gains from foundation-backed reforms could lead to the abandonment of promising practices before they have a chance to prove themselves. The challenge for foundations, then, is not to create the initiative that will work precisely as planned. The challenge is to use a variety of strategies--investment, capacity building, the creation of competition, parallel structures that challenge the status quo--to help the schools and districts themselves do a better job. (Contains 1 table and 1 figure.)