Over the last 50 years, exercise motivation has been assumed to arise from a conglomeration of cognitive appraisals (e.g. of anticipated benefits versus costs, social support, personal agency, autonomy). While research has established the importance of cognitive mediators, a considerable portion of the variance in exercise behavior remains unaccounted and intervention effects are typically modest and short-lived. Therefore, an expansion of the theoretical perspective is warranted. This chapter outlines the Affective-Reflective Theory (ART) of physical inactivity and exercise. The ART postulates that the motivation to choose exercise over inactivity reflects the constant interplay of (a) reflective processes that generate deliberate action plans by evaluating explicit propositional data, and (b) automatic processes that generate action impulses linked to past, pleasant or unpleasant, affective experiences associated with exercise and inactivity. The ART further predicts that, under conditions of abundant self-regulatory resources, the reflective system may effectively inhibit contrary action impulses (e.g. adhering to exercise despite displeasure); however, under conditions of diminished self-regulatory resources (e.g. under stress or time pressure), automatic action impulses (e.g. to avoid exercise and remain sedentary) may prevail. The ART is the first “post-cognitivist” theoretical model that considers the unique challenges inherent in the choice among exercise versus physical inactivity.