Global health security is concerned with the response to infectious disease outbreaks across international borders. However, its precise nature and scope are contested because of lack of agreement about its focus and beneficiaries. Informed by qualitative interviews with thirty-eight professionals working in the field, this article explores views on how the scope and boundaries of global health security should be understood. The findings illustrate the ways in which those working in global health security operate in a complex moral landscape, with a wide variety of definitional boundaries about what global health security means. Our thematic analysis of this contested area suggests that views on global health security can be helpfully understood in terms of four models, each with different conceptual and moral priorities. We label these four moral models as follows: threat model, collective model, reframing model, and humanitarian model. Understanding the field of global health security in terms of these four sometimes competing models provides a helpful way of mapping out and understanding the complex range of agreements and disagreements that arise in relation to epidemics and other global health security events.