Our understanding and perceptions of source credibility significantly drive how we process health and risk messages, and may also influence relevant behaviors. Source credibility is believed to be impacted by both perceptions of source trustworthiness and expertise, and the effect of credibility on changes in attitudes and behavior has been studied for decades in the persuasion literature. However, how we understand and define source credibility—particularly the dimension of expertise—has changed dramatically as social media and other online platforms are increasingly used to design and disseminate health messages. While earlier definitions of source credibility relied heavily on the source’s credentials as indicators of expertise on a given topic, more recent conceptualizations must also account for expertise held by laypeople who have experience with a health concern. This shifting conceptualization of source credibility may then impact both why and when people select, as well as how they perceive, process, and judge, health messaging across both novel and more traditional communication contexts.