The firearm, obesity, and opioid epidemics are among the most important public health crises of our time. Each epidemic has a complex etiology that challenges efforts at mitigation. From this, a central question arises for researchers, clinicians, and policymakers: How can we identify what matters most within a broad range of causal factors in these epidemics, and can we draw cross-epidemic inferences that will help inform our thinking?

The principles of population health science can shed light on the fundamental forces that drive each epidemic. Because population health science is a relatively new field, we do not yet have substantial agreement on a set of axioms to guide our work.1 Two years ago, building on the work of Geoffrey Rose,2 a colleague and I proposed 9 principles to guide the science of population health.3 These principles, presented in the box, offer a framework that can inform research on the drivers of population health. I focus here on 2 of these principles to illustrate how they can apply to—and help set priorities for—these 3 wide-ranging epidemics.