The Global Systems Simulator addresses the issues of sustainability and carrying capacity at a global scale - the same issues addressed by the Jay Forrester's World model. The current version of the GSS is a prototype intended as proof of concept for a much different approach to modeling. Systems models are seen as explicit extensions of the mental models we use to interpret the signals received by our sensory apparatus and to navigate in the real world - extensions that enable us to perceive the long term and systemic consequences of potential actions. The approach has its roots in the activity analysis of Koopmans, Leontief, and Georgescu-Roegan, the system dynamics of Forrester, the control theory of Mesarovic, the general system theory of Weiner and Laszlo, the principle of uncertainty of Prigogine, and the cognitive theory of Bateson, Maturana and Varella. The GSS is implemented using the whatIf? software technology, a platform developed by Robbert Associates for large scale simulation modeling. The workshop will demonstrate these concepts using the Global Systems Simulator.