People regularly make decisions about their health, yet they clearly differ in their ability to successfully make healthy decisions. We sought to understand this important individual difference by developing a scenario-based measure of health competence (HC) modeled from the Situation Judgment Test (SJT) method. People were required to judge certain responses to health-related scenarios in terms of how healthy the response was and the likelihood that they would enact the response. In study 1, we showed that those with high HC scores tended to participate in less risky health behaviors and more protective health behaviors. In study 2, we used a daily diary methodology to show that HC scores were predictive of daily substance use, healthy eating, impulsivity, and coping. These findings suggest that this HC assessment will contribute to our knowledge of how people make health decisions and how those decisions affect their health.