The ability to engage in avoidance behaviors is vital to adaptively navigating different environments, especially if these environments are unpredictable or hostile. Despite considerable advances in understanding the neural mechanisms that give rise to avoidance behaviors, some component processes remain controversial. This dissertation focuses on understanding the neural mechanisms in determining two elements of avoidance behaviors: value (whether a particular reward is worth whatever it takes to get it) and the valence (whether a stimulus or event is good or bad). The first part of this dissertation focuses on understanding the neural mechanisms of value reduction in context of different decision costs. Specifically, Chapter 2 examines the neural representations of delay, effort, and probability costs, which have all been shown to devalue rewards, thereby making the reward pursuit less likely. We used a computational meta-analysis to analyze the brain activations associated with the three types of costs reported neuroimaging literature over the past 15 years. We found that all 3 costs consistently engage dorsal striatum and anterior insula. We also found that delay and probability, but not effort costs, consistently engage prefrontal regions (BA46) associated with top-down control. The latter part of the dissertation focuses on understanding negative valence representations in the human brain. In chapter 3 we examined the neural correlates of negative valence across different type of aversive stimuli. In this study participants were subjected to painful pressure as well as emotionally and physically aversive sounds. We found that negative valence modulates activity in areas primarily associated with sensory processing, supporting the theory of modality-specific affect. In chapter 4 we investigated the role of emotion regulation in the processing of aversive stimuli. We found that trait emotion regulation ability (as measured by DERS questionnaire) did not modify subjective displeasure ratings of any aversive stimuli. In addition, DERS scores modulated brain activity in pressure pain trials during pressure pain anticipation and pain delivery, but had no effect on neural processing of emotionally aversive or physically aversive sounds. Specifically, we found that people with lower trait emotional regulatory capacity show less preparatory activity during pain anticipation and instead rely on dorsolateral prefrontal cortex to regulate pain.