In the present application, I am requesting 2 years of support for pre-doctoral research in clinical and experimental psychology. The proposed training plan involves regular meetings with my sponsor (Dr. Michelle Craske), co-sponsor (Dr. Michael Fanslow), and consultant (Dr. Bruce Naliboff), focused course load and training experiences that specifically address gaps in my research experience, and the completion of two related research projects. Background: Anxiety disorders and phobias represent the most common category of psychopathology, estimated to affect roughly 20% of individuals (Pine & Klein, 2008). Though exposure therapy is an effective means of reducing phobias and anxiety (e.g., Norton & Price, 2007; Tolin, 2010), many patients experience a return of fear following reduction of fear through treatment (Craske and Mystkowski, 2006). One cause of the return of fear is reinstatement, a behavioral phenomenon in which the presentation of an unconditional stimulus (US) alone following extinction produces a subsequent return of the conditioned response (CR) to the conditional stimulus (CS) (e.g., Pavlov, 1927; Rescora and Heth, 1975). Though human fear reinstatement has been well demonstrated in laboratory studies (Dirikx et al., 2004; Dirikx et al., 2007; Hermans et al., 2005, Neumann, 2008), there is a dearth of clinical research on which types of stimuli reinstate which types of fear, and through which processes they do so. While an associative account of reinstatement would suggest that only a US that is very similar to the original US should reinstate CR to the CS, an arousal-based account would suggest that any unpaired US that produces a UR similar to the internal state of arousal experienced during initial US-CS pairings should be sufficient to reinstate the CR. Additionally, though a substantial body of research indicates that the belongingness of CS with US significantly impacts fear learning, there is little evidence regarding the role of belongingness between CS and reinstating US. The gap in the literature pertaining to reinstatement of fear in humans indicates that elucidating the factors which contribute to post-treatment return of fear represents a crucial yet relatively nascent area of research which may inform the development of clinical strategies for offsetting reinstatement following exposure therapy. Research Plan: The broad goal of this proposal is to clarify the basic associative and arousal-based processes involved in reinstatement of conditional fear. Two experimental conditioning studies will be conducted comparing the reinstating effects of a physically painful US versus a socially threatening US. Study 1 will evaluate the extent to which the nature of a reinstating US affects reinstatement of conditioned fear, and the extent to which arousal experienced during reinstatement moderates the return of fear. Study 2 will attempt to replicate the findings from Study 1, and investigate th role of belongingness between a CS and a reinstating US on return of fear.