A system and method are provided for controlling power mode transitions. The system has a plurality of power domains, where each power domain has one or more components, and a plurality of power controllers, where each power domain is associated with one of the power controllers. For each power domain, the associated power controller controls transition of that power domain between a plurality of power modes. The power controllers are connected by communication links in order to implement a hierarchical relationship between the power controllers that comprises two or more hierarchical levels. Each power controller other than a highest level power controller in the hierarchical relationship is connected by a communication link to an associated higher level power controller at a higher hierarchical level in the hierarchical relationship. Each given power controller is arranged to implement a power mode transition policy in order to control transition of the associated power domain between the plurality of power modes. The power mode transition policy takes into account power mode information received from any other power controller that a given power controller is connected to by a communication link, so as to constrain the power mode transitions within the associated power domain in accordance with the hierarchical relationship. This provides an efficient and effective mechanism for handling transitions in power modes of power domains that are related to each other, but managed by different power controllers.