It has become increasingly clear in recent decades that lifestyle factors contribute significantly to the ever-growing burden of chronic disease. Two behavioral patterns in particular, diet and smoking, have especially potent effects on the health of individuals and communities. As our understanding of the factors that affect health improves, our health care system must align with this knowledge—researching, teaching about, and striving to reduce preventable illness. April’s contributors examine the roles physicians should play in encouraging healthy lifestyles and helping patients change their behaviors, creating a health-promoting environment, and expanding public awareness of disease prevention.