This study examined the developmental course and adjustment correlates of time with peers from age 8 to 18. On seven occasions over 8 years, the two eldest siblings from 201 European American, working- and middle-class families provided questionnaire and/or phone diary data. Multilevel models revealed that girls' time with mixed-/opposite-sex peers increased beginning in middle childhood, but boys' time increased beginning in early adolescence. For both girls and boys, time with same-sex peers peaked in middle adolescence. At the within-person level, unsupervised time with mixed-/opposite-sex peers longitudinally predicted problem behaviors and depressive symptoms, and supervised time with mixed-/opposite-sex peers longitudinally predicted better school performance. Findings highlight the importance of social context in understanding peer involvement and its implications for youth development. Copyright © 2014 The Authors. Child Development © 2014 Society for Research in Child Development, Inc.