Background/Context: Recent educational policy developments have sought to raise the academic rigor of students' high school experiences to increase student preparation for postsecondary education. The expansion of credit-based transition programs (CBTPs), both in number and in the type of student served, represents one such strategy. These programs allow high school students to take college classes and earn college credit while still in high school. Despite policy makers' enthusiasm, there has been little theorizing about why CBTPs might lead to improved student access to, and persistence in, college. Further, because any policy created to support these programs lacks a theoretical foundation, unintended consequences may result. Research Questions/Focus of Study: The research question guiding this study was, Through what mechanisms might credit-based transition programs encourage student success in postsecondary education? This article presents a conceptual model hypothesizing why and how CBTPs may lead to their intended outcomes. We then explore five CBTPs in diverse policy contexts. We describe the ways that programs are attempting to meet the needs of a wide range of students, and identify program features that appear to best prepare middle- and low-achieving students for postsecondary education. Research Design: Five in-depth qualitative case studies were conducted. Two visits were made to each site, during which we conducted interviews and observations with faculty, staff, and students, and collected supporting documents. Interview transcript and observational data were uploaded into NVivo, a qualitative software program, for coding and analysis. Conclusions/Recommendations: The case study data demonstrated that our initial conceptual model oversimplified program structure and the interaction among program components. The model was refined to reflect that complexity and to take student motivation into account. The final model hypothesizes that student participation in college coursework and support services, along with the attendant growth in academic skills, knowledge of the social aspects of college, and motivation, will lead students to matriculate into postsecondary education. Moreover, because of their strong skills, students will be likely to persist in college once there. Future research should seek to test this model. In the meantime, the findings have important implications for policy makers and educators because they suggest that middle- and low-achieving students may benefit from participation in CBTPs if they are properly prepared for, and supported in, their college courses. In addition, the findings stress the importance of collaboration and communication across secondary and postsecondary sectors.