Engaging in maladaptive health behaviors such as excessive alcohol intake and sexual risk taking behavior and avoiding adaptive health behaviors such as a healthy diet and exercise contribute to the development of a number of chronic illnesses. Rumination, thinking about things negatively over and over again, is a common cognitive process among college students that may impact health behavior engagement or avoidance. However, very little research has examined the relationship between rumination and health behaviors, and none to our knowledge has studied possible moderators and mediators. This study examined these relationships using an 11-day online daily diary design. Outcomes included fruit intake, vegetable intake, exercise, alcohol intake, sexual risk taking behavior, and cigarette smoking. Using multivariate modeling in Mplus, we found that significant Level 2 moderators included intention for all health behavior and perceived behavioral control and neuroticism for alcohol only. Emotional intelligence and mindfulness were surprisingly not significant moderators. Significant Level 1 mediators included impulsivity and using health behaviors as coping for most health behaviors, but motivation was a significant mediator for exercise only and self control was only a significant mediator for vegetable intake. We also found that individuals with anxiety symptoms may ruminate and then act impulsively and individuals with depressive symptoms may ruminate and then not act at all. We discuss implications of this research and application to interventions, including the potential limits of mindfulness and the potential importance of cognitive behavioral therapy and stress management focused interventions on college campuses. [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.]