A system comprises a first domain 4 and second domain 6 which communicate via an interface 8. The first domain 4 transmits power state commands to the second domain 6 for controlling transitions of power states at the second domain 6. The power state commands include at least a power up command 50 for triggering a transition to a power up state and a power no-operation command 52 in response to which the second domain remains in the current one of the power states. The no-operation command 52 enables the second domain 6 to be left in either the power up state or a different power state even if the first domain 4 is powered down.