This study explored the effects of service learning and community service on college undergraduates' cognitive and affective development, investigating how service enhanced learning. Longitudinal data were collected from 22,236 students attending a national sample of baccalaureate-granting institutions. Most had entered college as freshmen in fall 1994 and were followed up in fall 1998 percent. About 30 percent had participated in course-based community service during college; 46 percent participated in other forms of community service. The study assessed the impact of service learning and community service on 11 dependent measures (academic outcomes, values, self-efficacy, leadership, career plans, and plans to participate in further service after college). The qualitative portion of the study involved individual and group interviews with faculty and students on three different campuses. The study found that service participation showed significant positive effects on all 11 outcome measures. Service participation had its strongest effect on students' decisions to pursue service careers. Most students believed their service made a difference. Students' degree of interest in the subject matter was the most important factor associated with positive experiences. Both faculty and students developed a heightened sense of civic responsibility as a result of the experience. (Contains 13 references.) (SM)