The Behavioral Activation System (BAS) has been described as playing a central role in approach motivation and reward sensitivity (Gray, 1970). Self-report measures of BAS (e.g., Carver & White, 1994) have been used to index BAS activity, with higher scores interpreted as indicating greater BAS activity (e.g., Hundt et al., 2008). However, Beauchaine and colleagues (e.g., Brenner, Beauchaine, & Sylvers, 2005) have challenged this view, noting psychophysiological and neuroimaging evidence showing that externalizing behaviors are associated with reduced BAS functioning. Furthermore, global self-reported BAS scores are often used to index approach behavior, despite evidence that two main BAS traits, impulsivity and reward reactivity, are psychometrically distinct (Smillie et al., 2006). The present study tested a measurement model of these proposed components of BAS, as well as relationships between self-report and psychophysiological BAS indices. A large undergraduate student sample completed self-report indices (N=599) and a smaller subsample also completed psychophysiological (N=18) indices of BAS-related constructs. As hypothesized, a two-factor model with impulsivity and reward reactivity as separate, correlated constructs demonstrated better model fit than a one-factor alternative model. Associations between psychophysiological indices of BAS and indices of reward reactivity and impulsivity were mixed. Implications regarding future measurement of BAS and autonomic response to reward are discussed. / Master of Science