Working memory (WM) is crucial for planning, reasoning, and learning, and is one of the most extensively studied topics in cognitive psychology and neuroscience. However, the concept of a WM subsystem for motor content - or 'motor working memory' (MWM) - is generally neglected, even though MWM likely plays an important role in everyday action. Here, we synthesize evidence that the brain both prospectively and retrospectively maintains motor content in WM and propose that MWM carries out multiple key computational functions in motor control and skill learning. A focused research program on MWM is overdue and will deepen our understanding of the links between cognition and action.