Over the past decade, research on peer influence processes in adolescence has advanced significantly, with five key themes emerging: broadening the range of behaviors influenced by peers, distinguishing sources of influence, examining conditions that amplify or reduce influence, testing theoretical models of peer influence, and exploring behavioral neuroscience perspectives. Peer influence is defined as a process involving both selection (choosing peers with similar characteristics) and socialization (behavioral similarity over time). While previous research focused on deviant and aggressive behaviors, recent studies have expanded to include internalizing behaviors, academic motivation, and prejudiced attitudes. Peer influence is also relevant to healthy behaviors and can protect against maladaptive outcomes. Research has identified various peer sources of influence, including romantic partners, best friends, and larger peer networks. The mechanisms of peer influence include social learning, identity-based conformity, and social norms. Adolescents may conform to peers for status, social norms, or reinforcement. Peer influence can also lead to anticonformity when adolescents reject majority preferences. The effects of peer influence are moderated by factors such as the target adolescent's characteristics, the influencer's traits, the nature of the relationship, and the type of behavior. For example, adolescents with higher self-regulation are less susceptible to peer influence, while those with social anxiety may conform more easily. Peer influence is also influenced by biological factors, including genetic liability and brain development. Research using neuroimaging has shown that neural activity patterns differ between individuals who resist peer influence and those who are more susceptible. Behavioral genetics suggests that genetic factors interact with environmental influences to shape susceptibility to peer influence. Future research should focus on understanding ethnic and cultural differences in peer influence, longitudinal studies to examine reciprocal relationships between selection and socialization, and the integration of behavioral genetics, neuroscience, and peer influence research. These advances will help develop more effective interventions to reduce negative peer influence effects and promote healthy adolescent development.Over the past decade, research on peer influence processes in adolescence has advanced significantly, with five key themes emerging: broadening the range of behaviors influenced by peers, distinguishing sources of influence, examining conditions that amplify or reduce influence, testing theoretical models of peer influence, and exploring behavioral neuroscience perspectives. Peer influence is defined as a process involving both selection (choosing peers with similar characteristics) and socialization (behavioral similarity over time). While previous research focused on deviant and aggressive behaviors, recent studies have expanded to include internalizing behaviors, academic motivation, and prejudiced attitudes. Peer influence is also relevant to healthy behaviors and can protect against maladaptive outcomes. Research has identified various peer sources of influence, including romantic partners, best friends, and larger peer networks. The mechanisms of peer influence include social learning, identity-based conformity, and social norms. Adolescents may conform to peers for status, social norms, or reinforcement. Peer influence can also lead to anticonformity when adolescents reject majority preferences. The effects of peer influence are moderated by factors such as the target adolescent's characteristics, the influencer's traits, the nature of the relationship, and the type of behavior. For example, adolescents with higher self-regulation are less susceptible to peer influence, while those with social anxiety may conform more easily. Peer influence is also influenced by biological factors, including genetic liability and brain development. Research using neuroimaging has shown that neural activity patterns differ between individuals who resist peer influence and those who are more susceptible. Behavioral genetics suggests that genetic factors interact with environmental influences to shape susceptibility to peer influence. Future research should focus on understanding ethnic and cultural differences in peer influence, longitudinal studies to examine reciprocal relationships between selection and socialization, and the integration of behavioral genetics, neuroscience, and peer influence research. These advances will help develop more effective interventions to reduce negative peer influence effects and promote healthy adolescent development.