Thinking of a computer as an educational tool emphasizes a solitary interaction between the learner and the computer. A tool is something that is applied to an object in order to change it, and this usually implies a single user working on a single object or project. While tools are an important aspect of an educational experience, the tool metaphor can isolate individual learners from each other and from their teachers instead of cultivating a sense of educational community. Reconceptualizing the computer as a constructionist medium increases the computeršs educational value, by allowing the development and support of communities of learners. As a medium, the computer is viewed as a collection of tools and capabilities that are used to communicate. This reconceptualization leads to a new class of computer applications that places constructionist activities within a more social context in which computers simultaneously provide opportunities for learning how to communicate as well as for enabling communication to enhance the learning experience. Computers are a particularly interesting educational medium, because not only do they allow communication via more traditional media (e.g. text, pictures and video), but they also enable the communication of ideas through the creation and sharing of computational objects (e.g. agents, simulations and analysis tools). As these computational objects become easier to create, share and combine, the educational opportunities afforded by the computer become more viable and effective. This article describes the AgentSheets system and its use as a constructionist medium in K-12 and university educational settings. Specifically, AgentSheets supports the social creation of SimCityTM-like interactive simulations by providing an environment for the definition and sharing of computational components, called agents, through the World Wide Web.