The benefits of Head Start are under increased scrutiny as Congress debates its reauthorization. How effective is Head Start, and how can it be improved? We provide a current overview and critical evaluation of Head Start research and discuss implications of this research with an eye toward informing debate. There has been a good deal of controversy over whether Head Start produces lasting benefits, dating back to its early years. Our review finds mixed, but generally positive, evidence regarding Head Start's long-term benefits. Although studies typically find that increases in IQ fade out over time, many other studies also find decreases in grade retention and special education placements. Sustained increases in school achievement are sometimes found, but in other cases flawed research methods produce results that mimic fade-out. In recent years, the federal government has funded large-scale evaluations of Head Start and Early Head Start. Results from the Early Head Start evaluation are particularly informative, as study participants were randomly assigned to either the Early Head Start group or a control group. Early Head Start demonstrated modest improvements in children's development and parent beliefs and behavior. The ongoing National Head Start Impact Study, which is also using random assignment, should yield additional insight into Head Start's effectiveness. We conclude with suggestions for future research.