Perceptual-motor workspaces result from the interaction of constraints relating to the actor and constraints relating to the environment. The study of goal-directed behavior may be viewed as the systematic identification of these constraints under different task conditions. In man-machine systems the environmental constraints are defined in part by the plant properties of the machine (i.e., its control structure). Because a high degree of explicit control can be achieved over these properties, the use of machines in the study of the control of action allows for experimental manipulations that are impossible or very difficult to realize without them. The danger of this approach is that the plant properties may deviate so much from those of real environments, or are so constraining, that experimental results obtained in these environments have poor generalizability.