This document discusses, among other things, a modular antitachyarrhythmia therapy system. In an example, a modular antitachyarrhythmia system includes at least two separate modules that coordinate delivery an antitachyarrhythmia therapy, such as a defibrillation therapy. In another example, a modular antitachyarrhythmia therapy system includes a sensing module, an analysis module, and a therapy module.