Health information is increasingly being shared online not just by credentialed sources such as physicians or health organizations, but also by patients with personal experience with a health concern. This dissertation proposes a new measure of vigilance-avoidance, or tendency to approach or avoid threatening stimuli, in order to understand how individual differences in this personality characteristic might influence selection and processing of online information about health. Two online experiments focusing on the topic of breast cancer were conducted to explore how individual levels of vigilance-avoidance moderate the effects of health message threat, community endorsement (e.g., online recommendations), and source type (patient- or physician-generated information) on selective exposure, perceptions of credibility, and helpfulness ratings. Results of these studies indicate that vigilance-avoidance interacts with these source, message, and community characteristics to influence both online health information selection behaviors and evaluations of online health information. Women who are more vigilant spend more time reading threatening health information and evaluate it as more credible than women who are more avoidant, suggesting that they may be more likely to find health information more credible simply because it is negative. Additionally, women who are more avoidant spend more time reading low threat information than high threat information. Women who are more avoidant also spend even less time looking at credible health information from a physician than they do information from a patient, suggesting that when allowed to selectively expose themselves to information, they may miss key messages from credentialed medical sources that are important to their overall health. Overall, results from these studies suggest that our tendency to approach or avoid health information, as well as our potential to trust and find it credible, are dependent on these key personality characteristics--a novel contribution in the areas of health, selective exposure, and information evaluation research. [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.]