Research has shown that most STEM doctoral students are not prepared for their future careers. To address this gap, this dissertation explored and made sense of engineering doctoral student experiences related to their development as early career professionals with a variety of future career interests. One consideration embedded throughout the research is the default of graduate education is to train students for academic careers, such as tenure track faculty positions, despite nearly 75% of students being interested in non-academic careers. Through quantitative and qualitative methods, I found that students' development toward future careers is driven by student-specific (e.g., student future career interests) and programmatic factors (i.e., faculty advisor and graduate programs). Results from this work indicate that students were more likely to feel prepared when they have an internalized self-set reason for going to graduate school, have a specific future career goal, have a plan for reaching their future career goal, and receive feedback and support from faculty advisors and others in their program related to their future career goal. Recommendations for intervention guided by the data in this dissertation include encouraging students to explore their future goals, embedding career-aligned feedback and support into graduate programs, and encouraging multiple sources of mentoring to improve engineering doctoral students' perceptions of career preparation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]