Power management contracts for accessory devices are described. In one or more implementations, a power management contract is established for a system including a host computing device and an accessory device based at least in part upon power exchange conditions observed for the system. The power management contracts define operating constraints for power exchange between components of the system, including at least a power exchange direction and current limits. The host computing device and accessory devices are each configured to renegotiate the power management contract to dynamically change operating constraints in “real-time.” Additionally, different power management contracts may be associated with identifying data corresponding to different types of accessory devices.