Since 1993, over 500,000 people have served in AmeriCorps national service programs. This dissertation evaluates the long-term impacts of AmeriCorps service on participants, particularly in the areas of civic engagement, future volunteerism, appreciation of diversity, and a number of other job and life skills. It fills a gap by using both quantitative and qualitative methods to help illuminate some of the ways that program characteristics play a mediating role on the participants' outcomes. The analyses, which converge on a set of consequential program characteristics, have implications for program design as organizations try to create programs that will have positive and long-lasting impacts on members. The salient program features include support from mentors, strong relationships, focused training, leadership opportunities, projects that allow members to feel they have made a difference, and exposure to new and unique situations. Programs with team-based formats and diverse groups of people can have positive impacts, but diversity and teamwork must be structured with care. In addition, the interviews revealed that service can alter the life-course at key transition points and that exposure to new circumstances may have particularly strong impacts at pivotal times in young adults' development. Other program features did not emerge as significantly related to long-term impacts on participants. In the end, service programs are highly variable, and the specifics of the experience may ultimately determine whether or not a participant is impacted by the program. Five appendices are included: (1) Alumni interview protocol; (2) Details of interview participants; (3) Alumni interview codebook; (4) Survey items used in constructing outcomes; and (5) Quantitative analysis results. (Contains 10 footnotes.) [This document was submitted as a dissertation in August 2009 in partial fulfillment of the requirements of the doctoral degree in public policy analysis at the Pardee RAND Graduate School.]