In this inaugural “Policy Models in Pandemic” essay, part of the “Covid-19 and the Social Sciences” series, Michael Feuer broadly explains mathematical and theoretical models. Recent media coverage has brought the concept of “models” to the fore, as ways to predict and understand the spread of Covid-19. Feuer shows that models are fundamentally representations of complex phenomena, aimed at guiding rational action or providing useful information to decision makers. Since public policy and public health decisions are often made based on information from mathematical and theoretical models, he suggests it prudent to review the origins, purposes, benefits, and inherent imperfections of these models as well as their value.