Background: Increasing human resources in engineering is a key concern for the United States. While some research has considered pathways to doctoral study, there is no clear empirical evidence on the role of undergraduate experiences in motivating engineers to continue to graduate school, both in engineering programs and more broadly. Purpose/Hypothesis: We investigated three influences on engineers' decisions to enter graduate school: mathematics proficiency, self-assessments of engineering skills, and co-curricular experiences. Design/method: Using data from 1,119 engineers, we developed a hierarchical multinomial logistic model to examine engineers' graduate school enrollment patterns. Results: Math proficiency, participation in undergraduate research, and self-assessed leadership skills are significant positive predictors of attendance in an engineering graduate program, although self-assessed teamwork skills are a negative predictor. For attendance in a nonengineering graduate school program, math proficiency, nonengineering community volunteer work, and engineering clubs were positive predictors, but none of the self-assessed skills were significant predictors. Conclusions: Our findings support past research that emphasized academic preparedness in mathematics, and further corroborate the claim that K--12 math education is a key policy lever to the engineering pipeline from undergraduate to graduate education. Our findings also indicate differences between engineering and nonengineering graduate study in relation to self-assessed skills and co-curricular experiences. Future research is needed on which types of preparation during college are needed for graduate school choice.