Background: Engineering student support centers (ESSCs) are common providers of assistance for undergraduate students, especially those from underrepresented groups. However, ESSCs are relatively recent, and there are gaps in our knowledge about how they are intentionally designed to function alongside engineering curricula. Purpose/Hypothesis: The purpose of this article is to develop a conceptual model that graphically represents the practices of and intentions behind ESSCs. Design/Method: A multi-case study design was used to examine the efforts of six ESSCs housed at four institutions. Using qualitative methods, we collected data from student-support practitioners (those who provide support) and students (those who receive support). The data were analyzed through the development of a logic model. Results: The primary result of this study is an empirically based conceptual model, the model of co-curricular support (MCCS), for constructing and evaluating support systems and individual student interventions. The MCCS builds on Tinto's model of institutional departure and demonstrates the breadth of assistance required to comprehensively support undergraduate students at the college level. Conclusions: The MCCS indicates how a student's interactions with the academic, social, and professional systems in a college and the larger university system can influence a student's success in an undergraduate degree program. The model combines student-retention theory with student-support practice in a way that can facilitate future collaborations between educational researchers and student-support practitioners.