Peer influence is a twofold process that entails a behavior by an agent of influence that elicits conformity from the target of influence. Susceptibility describes the likelihood that conformity will occur. This review focuses on factors that shape susceptibility to peer influence. We argue that conformity has two distinct sources. In some instances, conformity is a product of characteristics of the target of influence, operationalized as stable individual difference variables. Trait-like attributes associated with susceptibility to peer influence include conformity dispositions, social goals, resource acquisition strategies, vulnerabilities, and maturational status. In other instances, conformity is a product of the context in which the target is situated, operationalized as impermanent individual difference variables. State-like circumstances associated with susceptibility to peer influence include conditions of uncertainty, personal attributes that differ from the partner or group, perceived benefits of impression management, unmet social needs, and social referents and beliefs about their behavior. Empirical illustrations are provided. We close with a discussion of developmental changes hypothesized to impact variations in susceptibility to peer influence.