This free, open source simulation tool, the outgrowth of physics education research, was developed as a means for students to visualize the underlying concepts necessary to approach and solve heat transfer problems. The author's stated goal is "to develop a free interactive computational tool that can run simulations in real time to provide students with a powerful online learning environment for the subject of heat transfer. To achieve this goal, our computational engine must be reasonably speedy and stable." The Energy2D package models only two-dimensional systems, thus is not intended to replicate commercial engineering software used in many university settings. Rather, it was constructed to allow for rapid experimentation through use of dynamic graphics that users can easily manipulate and comprehend. The sequenced materials include: heat and temperature, conduction, convection, radiation, Stefan-Boltzman Law, fluid dynamics, boundary conditions, and the heat equation. Registered users may capture, store and share data, and gain access to free authoring tools to customize or create new models. This item is part of the Concord Consortium, a nonprofit research and development organization dedicated to transforming education through technology. The Consortium develops deeply digital learning innovations for science, mathematics, and engineering. The models are all freely accessible.