As part of the High School and Beyond study, students in a large nationally-representative sample were asked whether or not they had participated in each of a variety of extracurricular activities in their sophomore year (1980) and again in their senior year (1982) of high school. Data for 1984--2 years after the subjects graduated from high school--were included. Out of 10,613 students, a sample size of 4,000 was assumed for the purposes of statistical testing. After controlling background variables and sophomore outcomes, total participation was significantly and favorably related to 17 of 22 senior and postsecondary outcomes (e.g., social and academic self-concept, educational aspirations, coursework selection, homework, absenteeism, academic achievement, and subsequent college attendance). However, there were significant non-linear components to most relations indicating that participation in too many activities produced diminishing returns. The benefits of participation also differed substantially depending on the particular activity. Participation in sport, honor societies, student government, school publications, school subject-matter clubs, church organizations, and community service organizations was consistently beneficial, but participation in some activities had mixed or predominantly negative effects. A 39-item list of references and eight data tables are included. (Author/TJH)