Dysfunctional affective processes are central to the experience of internalizing disorders (e.g., depression, anxiety, and related disorders). Specifically, extreme positive affect and elevated negative affect each have unique and robust patterns of associations with internalizing symptoms. This article examines affect as both an individual difference and a within-person dynamic process that unfolds over time. Recent research is reviewed that clarifies the hierarchical structure of affect and facet-level associations with symptoms, affect-laden traits that confer risk for internalizing psychopathology, models of emotion regulation, and how emotion regulation abilities and strategies contribute to or detract from psychological well-being. Several measurement challenges in this literature are identified and discussed, including possible conceptual and content overlap, mood-state distortion, naturalistic assessment in daily life, and the benefits and limitations of self-reported affective experience. (PsycINFO Database Record (c) 2018 APA, all rights reserved).