Individuals with health insurance use more health care. One reason is that health care is cheaper for the insured. Additionally, having insurance can encourage unhealthy behavior via moral hazard. Previous work studying the effect of health insurance on medical utilization has mostly ignored behavioral changes due to having health insurance, and how that in turn affects medical utilization. This paper investigates the structural causal relationships among health insurance status, health behavior, and medical utilization theoretically and empirically, and separates price effects from behavioral moral hazard effects. Also distinguished are the extensive versus intensive margins of insurance effects on behavior. (JEL C51, I12, D12)