A realm management unit (RMU) manages ownership of memory regions by realms, each realm corresponding to at least a portion of a software process executed by processing circuitry. Memory access circuitry enforces ownership rights for the regions, with the owner realm having a right to exclude other realms from accessing data stored within its owned region. The memory access circuitry permits execution, from within a current realm, of program code stored in a target memory region having an owner realm other than the current realm, when the target memory region is owned by a code realm and a code realm authorisation table 908 stored in at least one memory region owned by the current realm indicates that execution of program code from the target memory region is permitted by the current realm.