He is an epidemiologist, health services researcher and primary-care internist. His research focuses on patient and physician interventions to improve chronic disease prevention, and the mechanisms by which physicians adopt new prevention practices. Many of his published studies have documented and raised concerns about the so-called "quality gap" -- the healthcare system's failure to consistently implement clinically proven therapies -- and have helped shape policy initiatives aimed at improving medical care. His research has also focused on drug costs and patterns of medication prescribing. At the Stanford Prevention Research Center, he directs the Program on Prevention and Outcomes Practices. He maintains clinical responsibilities at Stanford's Preventive Cardiology and Internal Medicine clinics, and serves on Stanford Medical School's faculty Senate.