The positive impact of undergraduate research experiences on students' post-secondary success is well-documented. However, these conclusions are drawn from undergraduate students who already participate; very little research has explored the pathways by which students enter these experiences. Using data from a multi-institutional survey, we examined students' reasons for participating and differences across institutions and demographic groups. Overall, students cited social and experiential reasons as key motivators for participation and a perceived lack of research readiness as a key barrier. Differences were also found across academic year. Implications from this study address issues of access, preparation, and institutional policies around undergraduate research.