In response to the student retention and diversity issues that have been persistent in undergraduate engineering education, many colleges have developed Engineering Student Support Centers (ESSCs) such as Minority Engineering Programs (MEPs) and Women in Engineering Programs (WEPs). ESSCs provide underrepresented students with co-curricular support using student interventions in the form of programs, activities, and services. However, ESSCs have a relatively short history and there are gaps in our knowledge about these support systems. While the practice of providing students with co-curricular support has been evaluated, theories of co-curricular support have not been as thoroughly investigated; we know very little about how co-curricular support functions alongside engineering curricula. In an effort to help close the gaps in current literature, the purpose of my study was to explore how the student interventions offered alongside engineering curricula influence the undergraduate experience. To address this purpose, I used a multi-case study design to explore the particulars of six ESSCs housed at four institutions. I focused on the ESSC administrators (those who provide support) and undergraduate students (those who receive support) using multiple qualitative data collection methods. The primary result of this study was the Model of Co-curricular Support (MCCS), which is a version of Tinto's Model of Institutional Departure that I repurposed to demonstrate the breadth of co-curricular assistance required to comprehensively support undergraduate engineering students. The MCCS illustrates how a student's interaction with the academic, social, and professional systems within a college–as well as the university system surrounding the college–could influence the success he or she has in an undergraduate engineering program. More specifically, the MCCS is a conceptual model for constructing and evaluating support systems and individual student interventions that prioritize undergraduate engineering students. Within my study, I also identified several classifications of ESSCs and highlighted some pros and cons associated with various classifications and configurations. Ultimately, this research combines student-retention theory with student-support practice in a way that could facilitate future collaborations among educational researchers and student-support practitioners.