In the nearly fifty years since the adoption of the Higher Education Act of 1965, financial aid programs have grown in scale, expanded in scope, and multiplied in form. As a result, financial aid has become the norm among college enrollees. Aid now flows not only to traditional college students but also to part-time students, older students, and students who never graduated from high school. Today aid is available not only to low-income students but also to middle- and even high-income families, in the form of grants, subsidized loans, and tax credits. The increasing size and complexity of the nation's student aid system has generated questions about effectiveness, heightened confusion among students and parents, and raised concerns about how program rules may interact. In this article, Susan Dynarski and Judith Scott-Clayton review what is known, and just as important, what is not known, about how well various student aid programs work. The evidence, the authors write, clearly shows that lowering costs can improve college access and completion. But this general rule is not without exception. First, they note, the complexity of program eligibility and delivery appears to moderate the impact of aid on college enrollment and persistence after enrollment. Second, for students who have already decided to enroll, grants that tie financial aid to academic achievement appear to boost college outcomes such as persistence more than do grants with no strings attached. Third, compared with grant aid, relatively little rigorous research has been conducted on the effectiveness of student loans. The paucity of evidence on student loans is particularly problematic both because they represent a large share of student aid overall and because their low cost (relative to grant aid) makes them an attractive option for policy makers. Future research is likely to focus on several issues: the importance of program design and delivery, whether there are unanticipated interactions between programs, and to what extent program effects vary across different types of students. The results of this evidence will be critical, the authors say, as politicians look for ways to control spending. (Contains 2 figures, 1 table, and 77 endnotes.)