At issue is the school-to-prison pipeline--a term often used to describe the connection between exclusionary punishments like suspensions and expulsions and involvement in the criminal justice system. Black and Hispanic students are far more likely than white students to be suspended or expelled, and Black and Hispanic Americans are disproportionately represented in the nation's prisons. Is there a causal link between experiencing strict school discipline as a student and being arrested or incarcerated as an adult? Research shows that completing more years of school reduces subsequent criminal activity, as does enrolling in a higher-quality school and graduating from high school. Yet there is little evidence on the mechanisms by which a school can have a long-run influence on criminal activity. To address this, the authors examined middle-school suspension rates in Charlotte-Mecklenburg Schools, where a large and sudden change in school-enrollment boundary lines resulted in half of all students changing schools in a single year. They estimate a school's disciplinary strictness based on its suspension rates before the change and use this natural experiment to identify how attending a stricter school influences criminal activity in adulthood. Their analysis shows that young adolescents who attend schools with high suspension rates are substantially more likely to be arrested and jailed as adults. These long-term, negative impacts in adulthood apply across a school's population, not just to students who are suspended during their school years. Students assigned to middle schools that are one standard deviation stricter--equivalent to being at the 84th percentile of strictness versus the mean--are 3.2 percentage points more likely to have ever been arrested and 2.5 percentage points more likely to have ever been incarcerated as adults. They also are 1.7 percentage points more likely to drop out of high school and 2.4 percentage points less likely to attend a 4-year college. These impacts are much larger for Black and Hispanic male students. The authors also find that principals, who have considerable discretion in meting out school discipline, are the major driver of differences in the number of suspensions from one school to the next. The findings show that early censure of school misbehavior causes increases in adult crime--that there is, in fact, a school-to-prison pipeline. Further, they find that the negative impacts from strict disciplinary environments are largest for minorities and males, suggesting that suspension policies expand preexisting gaps in educational attainment and incarceration. There is some limited evidence of positive effects on the academic achievement of white male students, which highlights the potential to increase the achievement of some subgroups by removing disruptive peers. However, any effort to maintain safe and orderly school climates must take into account the clear and negative consequences of exclusionary discipline practices for young students, and especially young students of color, which last well into adulthood.