It can be very useful to organise a program such that it is constituted by a number of concurrently executing processes. In general this is the case when the program activities can be grouped into a number of specific tasks that can perform concurrently, in a way that their interactions can be minimised. Such a situation occurs when the program must control several external activities (e.g. storage devices, intelligent peripherals, and telephone calls). If such external activities can be handled independently, then it is advantageous to let each of them be controlled by a single process.