The mechanistic nature of computation is a recurring theme in logic, philosophy, and cognitive science. Gualtiero Piccinini (2007, 2012, 2015) and Marcin Milkowski (2013) develop this theme into a mechanistic theory of computing systems. A functional mechanism is a system of interconnected components, where each component performs some function within the overall system. Mechanistic explanation proceeds by decomposing the system into parts, describing how the parts are organized into the larger system, and isolating the function performed by each part. A computing system is a functional mechanism of a particular kind. On Piccinini’s account, a computing system is a mechanism whose components are functionally organized to process vehicles in accord with rules. Echoing Putnam’s discussion of multiple realizability, Piccinini demands that the rules be medium-independent, in that they abstract away from the specific physical implementations of the vehicles. Computational explanation decomposes the system into parts and describes how each part helps the system process the relevant vehicles. If the system processes discretely structured vehicles, then the computation is digital. If the system processes continuous vehicles, then the computation is analog. Milkowski’s version of the mechanistic approach is similar. He differs from Piccinini by pursuing an “information-processing” gloss, so that computational mechanisms operate over information-bearing states. Milkowski and Piccinini deploy their respective mechanistic theories to defend computationalism.