Students starting at a two-year college are much less likely to graduate than similar students who start at a four-year college, but the sources of this attainment gap are largely unexplained. This paper investigates the attainment consequences of sector choice and peer quality among recent high school graduates. Using data on all Preliminary SAT (PSAT) test-takers between 2004 and 2006, we develop a novel measure of peer ability for most two-year and four-year colleges in the United States—the average PSAT of enrolled students. We document substantial variation in this measure of peer quality across two-year colleges and nontrivial overlap between the two-year and four-year sectors. We find that half the gap in bachelor's degree attainment rates across sectors is explained by differences in peers, leaving room for structural barriers to transferring between institutions to also play an important role.