Community colleges serve nearly half of all undergraduates, including a disproportionate share of African American and Hispanic students, and are a key point of access to higher education for students who face socioeconomic disadvantages or other obstacles to participation in higher education. Community colleges stand to play a pivotal role in strengthening the nation's workforce and increasing opportunity in science, technology, engineering, and math (STEM) fields. However, a troublingly large fraction of community college students who complete advanced STEM courses ultimately leave college without postsecondary credentials. Utilizing data from the California Community College system, we investigated four hypothesized explanations for why students who have demonstrated potential to succeed in STEM fields by completing an advanced course in math, chemistry, or physics did not complete college. We found that students who left college tended to enter the STEM curriculum at lower levels of skills, struggled in non-STEM coursework, and did not take or were unable to pass STEM courses in other fields. They also were more likely to be White and more likely to be male. We conclude with recommendations for institutional policy and practice and for future research.