In the first decade of the 21st century, citizenship is increasingly channeled into and organized through digital communications and experiences (Center for Technology in Government, 1999; Fountain, 2001; Larsen & Rainie, 2002). This article presents an overview of a set of new information technologies—online modeling, simulation, and decision-support technologies that have the potential to transform the way citizens understand complex issues and communicate that understanding to fellow citizens and governmental decision makers. Those who have thought deeply about democracy and technology tend to understand that new technologies often possess both beneficial and detrimental traits. These traits are also likely to occur with respect to simulation technologies for citizen participation (Shapiro, 1999; Sunstein, 2000). Whether simulation technologies will ultimately be more beneficial than detrimental will ultimately be determined by how the next generation of simulation designers do their jobs. One purpose of this article is to identify factors that these designers should consider. What defines and makes simulation technologies unique is their ability to act as a new mode of communication. Models, simulations, and decision-support technologies communicate as much through experience as they do through any direct effort to convey a specific message. As Marshall McLuhan (1967) suggested, things are different when the medium becomes a major part of the message. In the case of models, simulations, and decision-support technologies, the medium can be much more than the video presentations that McLuhan identified as having such an impact on our culture. Specifically, these technologies can exist as entire miniature worlds of experience and can possess features that address multiple senses at one time. Moreover, simulations can be designed to specifically stimulate higher cognitive functions as well as our sense of discovery and history. Although the terms model, simulation, and decision support are similar (see Key Terms) in that all of them involve some use of representations to help human beings understand processes, each term possesses some connotations that can be useful in different circumstances. For example, simulations seems to imply a greater level of experience on the part of the user when compared to models and decision-support technologies. Similarly, the term decision-support technologies (or expert systems) tend to imply a more goal-oriented representation of a problem. Although it is important to understand the subtle differences among these terms, for the purposes of this article I will use the term simulation to refer to the entire spectrum of these technologies. Scientists and engineers have long used computer modeling, simulation, and decision support, but only recently have these technologies been employed in support of citizen participation in policy development. This article describes the uses of, the rationales for, and the trends behind the employment of these simulation technologies in this manner. Specifically, I examine three major factors that are driving the trend toward the use of simulations to promote citizen participation as well as to identify factors that will enable citizens and public managers to make better use these technologies.