What is it that unifies the control task in all its manifestations, from the thermostat to the operator of a nuclear power plant? At the same time, how do we explain the variety of the solutions that we see for the task? I propose a Knowledge Level analysis of the task which leads to a task-structure for control. Differences in availability of knowledge, the degree of compilation in the knowledge to map from observations to actions, and properties required of the solutions together determine the differences in the solution architectures. I end by discussing a number of heuristics that arise out of the Knowledge Level analysis that can help in the design of systems to control the physical world.