Despite abundant anecdotes and theories suggesting a causal effect of teachers' expectations on student outcomes, documenting its presence and size has been challenging. The reason is simple: positive correlations between what teachers expect and what students ultimately accomplish might simply result from teachers being skilled observers. In practice, distinguishing between accurate and biased expectations is difficult, because both teacher expectations and student outcomes are likely influenced by factors that researchers are unable to observe. Anecdotally, the authors believe that teachers' expectations are important. But just how important might they be? And could differences in teachers' expectations of white students and black students help to explain gaps in key outcomes such as college enrollment and completion? To explore these questions, the authors analyze the federal Education Longitudinal Study of 2002, which followed a cohort of 10th-grade students for a decade. Among other questions, the students' teachers were asked whether they expected their students to complete a four-year college degree. The authors use these responses to first document the presence of racial bias in teachers' expectations, and then study the effects of differences in teacher expectations on students' likelihood of completing college. The analysis supports the conventional wisdom that teacher expectations matter.