The principles and practices of structured programming have been expounded and illustrated by relatively small examples (Dahl, Dijkstra, Hoare, 1972). Systematic methods for the construction of parallel algorithms have also been suggested (Dijkstra, 1968, a, b). This paper attempts to extend structured programming methods to a program intended to operate in a parallel environment, namely a paging system for the implementation of virtual store. The design decisions are motivated by considerations of cost of effectiveness. The purpose of a paging system is taken to be the sharing of main and backing store of a computer among a number of users making unpredictable demands upon them; and to do so in such a way that each user will not be concerned whether his information is stored at any given time on main or backing store. For the sake of definiteness, the backing store is taken here to be a sectored drum; but the system could readily be adapted for other devices. Our design does not rely on any particular paging hardware, and it should be implementable in any reasonable combination of hardware and software. Furthermore, it does not presuppose any particular structure of virtual store (linear, two-dimensional, ‘cactus’, etc.) provided to the user program.