Emerging adulthood, a developmental stage typically bound by the ages between 18-25 years, is a period characterized by new autonomy and decision-making responsibilities, as well as the introduction of a number of weight-related risks, including physical inactivity. Given that behavior plays a key role in the prevention of many chronic health conditions, there is a strong need to understand the bridge between moderate vigorous physical activity (MVPA) intention and MVPA behavior. In this experimental study, two techniques for eliciting behavior change are combined and examined: (1) Mental Contrasting (MC), a problem-solving self-regulatory strategy through which individuals positively fantasize about a future-oriented wish and then imagine current obstacles in their present reality that stand in the way of their future-oriented wish, and (2) Implementation Intentions (II), a self-reflection strategy outlining action plans, and determining when, where, and how an intended behavior will be completed. Eighty one (N= 81) undergraduate students (82.7% women, mean age = 18.91, years, SD = 1.49) were randomized to either an MCII condition, active control condition, or passive control condition to examine impact of the combined MCII task on motivational-affective (goal commitment, expectancy beliefs, energization, positive affect, and decreased perceived barriers) and behavioral outcomes (daily and weekly minutes of MVPA) over a two week period. Compared to participants in the control conditions, I found that participants in the MCII intervention condition reported greater increases in weekly MVPA over the two weeks of the study; and, there was no significant change in daily MVPA over the study period and difference by condition. Further, I found significant between-person effects for greater barriers and greater perceived goal success predicting daily MVPA and significant within-person effects for greater positive affect and greater perceived goal success predicting daily MVPA. To gain deeper insights into these quantitative findings, I conducted qualitative exit interviews with four female participants with the aim of better understanding the feasibility and acceptability of the MCII task. This research expands current knowledge on using MCII to promote physical activity engagement in young adult college students using an experimental medicine framework. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]