Mathematical concepts are paragons of abstraction. The actual practice of mathematics, though, is decidedly concrete, material, one might even say fleshy. This raises the question, What is the body doing, when the body is doing mathematics? We answer by analyzing a video corpus of mathematicians in their natural habitat: at the blackboard, chalk in hand. Mixing qualitative and quantitative analyses, we describe systematically the ways mathematicians use their bodies to write, move, and gesture. Some surprises arise, such as the observation that mathematicians point nearly constantly but seldom produce representational gestures; and that they spend most of their time away from the blackboard at which they are writing. We discuss implications for creativity, mathematical cognition, and theories of embodied and distributed cognition more broadly.