Dual-process theories assume that there are two information processing systems that regulate human behavior: A reflective and an automatic system. Most physical activity (PA) intervention is grounded mainly on the reflective system that relies on individuals' intentions and regulatory goals or beliefs. Yet, such interventions are not leading to significant and sustained changes in PA. There is a growing body of research now focusing on the automatic regulatory system that occurs outside of intent. Automatic regulatory processes, however, are conceptualized far less in PA behavior. The broad aim of this dissertation is to expand our current understanding of automatic regulatory processes while accounting for dynamic aspects of PA, reflective processes, and individual differences. This dissertation consisted of three separate studies. The first study examined the automatic and reflective processes in relation to four different components of the PA domain: exercise, light and moderate to vigorous intensity non-leisure time PA (L-NLTPA and MV-NLTPA), and sedentary behavior (SED). AE of exercise was significantly correlated with moderate-to-vigorous PA (MVPA; min/d) and total PA (total activity counts [tac]/d), particularly in individuals with high PA intention and high self-efficacy. Executive function and personality indices (behavioral avoidance (or inhibition)/approach system [BIS/BAS] scale) significantly moderated the relationship between AE and PA behaviors. The second study aimed to investigate the stability of AE measures. Participants performed a computerized Single- Category Implicit Association Task (SC-IAT) twice, separated by 7-10 days. Internal consistency and test-retest reliability of the SC-IAT for the four PA domains (Exercise, L-NLTPA, MV-NLTPA, and SED) were evaluated. SC-IAT for the exercise domain showed acceptable internal consistency and test-retest reliability. In study 3, we investigated automatic approach-avoidance tendencies in common scenarios where individuals are faced with decisions to approach or avoid energy conserving or energy expending activities. Our results indicate that individuals responded faster to energy conserving cues than energy expending cues. In summary, we found that AE and reflective processes synergistically predict PA in active, young adults. Individuals are also more sensitive to energy conserving than energy expending stimuli.