There has been, to date, little discussion about the defining features and measures of wildlife health in the literature or legislation. Much wildlife health work focuses on the detection and response to infectious or parasitic diseases; this perspective has been reinforced by the focus of the One Health initiative on wildlife as sources of emerging infections. The definition of health as “the absence of disease” lags 70 yr behind modern concepts of human health and emerging concepts of wildlife health in terms of vulnerability, resilience, and sustainability. Policies, programs, and research that focus on the integration of wildlife health with natural resource conservation, ecosystem restoration, and public health need a working definition of health that recognizes the major threats to fish and wildlife are the result of many other drivers besides pathogens and parasites, including habitat loss, globalization of trade, land-use pressure, and climate change. A modern definition of wildlife health should emphasize that 1) health is the result of interacting biologic, social, and environmental determinants that interact to affect capacity to cope with change; 2) health cannot be measured solely by what is absent but rather by characteristics of the animals and their ecosystem that affect their vulnerability and resilience; and 3) wildlife health is not a biologic state but rather a dynamic social construct based on human expectations and knowledge.