Most real-world models contain many interacting (positive and negative) feedback loops. Some feedback loops are “natural” and cannot be readily managed or controlled (e.g., water evaporating from a pond is an example of a process that forms a negative feedback loop). Others, however, are “intentional” negative feedback loops that are used for active feedback control of a system. That is, these are feedback loops that are part of the design of the system, and are used to make the system behave in a specified manner. A heating system controlled by a thermostat is the classic example of such feedback control. Pumping water to maintain a target water level is a simple water resource example. We discussed using GoldSim to represent such systems in our January webinar.