The Spatio-Temporal Epidemiological Modeler (STEM) is a tool designed to help scientists and public health officials create and use models of emerging infectious diseases. STEM uses mathematical models of diseases (based on differential equations) to simulate the development or evolution of a disease in space and time (e.g., avian flu or salmonella). These models could aid in understanding, and potentially preventing, the spread of such diseases. STEM also comes pre-configured with a vast amount of reference or denominator data for the entire world. By using and extending the data and models in STEM it is possible to rapidly prototype and test models for emerging infectious disease. STEM also provides tools to help you compare and validate your models. As an open source project, the ultimate goal of STEM is to support and encourage a community of scientists that not only use STEM as a tool but also contribute back to it. STEM is designed so that models and scenarios can be easily shared, extended, and built upon.