This study estimates peer effects on alcohol use, drawing from a database of about 2000 randomly-assigned roommates on a college campus. The estimation of peer influences also takes into consideration ego's history of alcohol use and friendship with the peer. College students averaged an additional two-fifths of a binge drinking episode per month and an additional one-half of a drinking episode per month when randomly assigned a roommate who drank in high school than when assigned a roommate who did not drink in high school. An individual's prior history of alcohol use proves important. Peer effects on binge drinking as well as drinking for those who already drank in high school were about twice as large as average peer effects. When one did not have a history of alcohol use, negative peer influences were absent. Also important is the friendship between peers. When a peer is considered a best friend, the step-up effect (or positive interaction effect) increased by 1.25-1.61 drinking episodes per month. However, even when a peer is not considered a best friend, a drinking peer still increased ego's drinking episodes by 0.75-1.00 per month.