In this paper, I make a case for the modularity of the motor system. I start where many do in discussions of modularity, by considering the extent to which the motor system is cognitively penetrable, i.e., the extent to which its processing and outputs are causally influenced, in a semantically coherent way, by states of central cognition. I present some empirical findings from a range of sensorimotor adaptation studies that strongly suggest that there are limits to such influence under certain conditions. These results cry out for an explanation. In the remainder of the paper, I provide one: The motor system is cognitively penetrable, but nonetheless modular along broadly Fodorian lines, insofar as it is informationally encapsulated. This means that its access is limited to its own proprietary database in computing its function from input to output, which does not include the information stored in central cognition. I then offer a model of action control, from distal intention to action outcomes, that further helps to illustrate this picture and can accommodate the target empirical findings.