Substance use during adolescence is of particular concern because it is known to be associated with many undesirable outcomes. When an adolescent discovers that a peer is using substances, he or she is faced with a decision regarding the response to be taken (e.g., use substances with the peer, report it to authorities, tell the peer to stop). Available literature has given little consideration to this issue; therefore, the current study sought to examine the response of adolescents to discovering that a peer is using substances, within an ethnically diverse sample of 139 students from a public high school located in Florida. Since responses taken likely vary based on adolescents' own personal traits and characteristics, this study investigated how adolescents' gender, ethnicity, socio-economic status, grade level, and own use or non-use of substances were related to their response to discovering that a peer is using substances. Findings revealed that those students that reported personal marijuana use were more likely to report that they would respond to peer substance use in an undesirable way (e.g., use with the peer, do nothing), and less likely to take a positive action of any sort (e.g., discuss the peer's substance use with a trusted adult, tell the peer to stop). A second purpose of this study was to examine whether or not an adolescent's relationship with the peer using drugs or alcohol (specifically, close friend versus classmate) was related to the action the adolescent takes in response to the peer's substance use. Findings revealed that overall students reported a higher likelihood that they would take a positive action of some sort if the peer using substances was a close friend than a classmate. More specifically, more students reported that they would tell a close friend to stop using substances than tell a classmate the same thing. On the other hand, students also reported that they would be more likely to use substances with a close friend than with a classmate. Implications of these findings for future research and practice are discussed.