Models provide a powerful learning tool for students in a variety of subjects and grade levels. Multiple representations of complex concepts and phenomona help students understand them in a deep and meaningful way. There are models of molecules interacting, of social behavior, of the solar system, of climate changes, and of the human body. Computers and the Internet provide teaches with new models and tools for helping students visualize and understand subjects through exploration and active learning. The Web also offers numerous resources for using computer-based visualization in the K-12 classroom.