This study tested whether pro-alcohol peer influences and prosocial involvement account for increases in drinking during the transition into emerging adulthood and whether these mechanisms differ depending on college attendance and/or moving away from home. The authors used structural equation modeling of prospective data from 825 young men and women. For 4 groups defined by college and residential status, more drinking in the spring of 12th grade predicted more pro-alcohol peer influences the following fall, and more pro-alcohol peer influences in the fall predicted increases in drinking the following spring. Going to college while living at home was a protective factor against increases in drinking and selection of pro-alcohol peer involvements. Prosocial involvement (measured by involvement in religious activities and volunteer work) was not significantly related to post-high school drinking except among college students living away from home. Prevention efforts should focus on (a) reducing opportunities for heavy drinking for college and noncollege emerging adults as they leave home and (b) increasing prosocial involvement among college students not living at home.