Background: Longstanding data have established that women earn about 20% of undergraduate degrees in engineering; they also have lower academic self-efficacy in the science, technology, engineering, and math fields than do men. This study examines these findings through a longitudinal design that explores whether cooperative education (co-op) can improve the retention of women (and men) in their undergraduate studies. Purpose: This study examines the effect on retention of demographic characteristics, cooperative education, contextual support, and three dimensions of self-efficacy -- work, career, and academic -- and their change over time. It incorporates longitudinal measures and a data check at the end of the students' fifth year. Design/Method: Respondents filled out 20-minute surveys, approximately one year apart, during three separate time periods. The study introduced and validated a number of new scales. The data for each time period were submitted to successive analyses. Results: The findings verified the study's pathways model. Academic achievement and academic self-efficacy, as well as contextual support in the case for women, in all time periods were critical to retention. Work self-efficacy, developed by students between their second and fourth years, was also an important factor in retention, although it was strongly tied to the students' participation in co-op programs. Higher retention was associated with an increased number of co-ops completed by students. Conclusion: Relationships between work self-efficacy and co-op participation and between academic self-efficacy and academic achievement play a critical role in retention for both male and female students.