In computer operating systems, memory paging is a memory management scheme that allows the physical memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compaction to reduce fragmentation.

Paging is often combined with the related technique of allocating and freeing page frames and storing pages on and retrieving them from secondary storage in order to allow the aggregate size of the address spaces to exceed the physical memory of the system. For historical reasons, this technique is sometimes referred to as swapping.

When combined with virtual memory, it is known as paged virtual memory. In this scheme, the operating system retrieves data from secondary storage in blocks of the same size (pages). Paging is an important part of virtual memory implementations in modern operating systems, using secondary storage to let programs exceed the size of available physical memory.

Hardware support is necessary for efficient translation of logical addresses to physical addresses. As such, paged memory functionality is usually hardwired into a CPU through its Memory Management Unit (MMU) or Memory Protection Unit (MPU), and separately enabled by privileged system code in the operating system's kernel. In CPUs implementing the x86 instruction set architecture (ISA) for instance, the memory paging is enabled via the CR0 control register.

In the 1960s, swapping was an early memory management technique. An entire program or entire segment would be "swapped out" (or "rolled out") from Random-access memory (RAM) to disk or drum, and another one would be swapped in (or rolled in). A swapped-out program would be current but its execution would be suspended while the RAM was in use by another program; a program with a swapped-out segment could continue running until it needed that segment, at which point it would be suspended until the segment was swapped in.

A program might include multiple overlays that occupy the same memory at different times. Overlays are not a method of paging RAM to secondary storage but merely of minimizing the program's RAM use. Subsequent architectures used memory segmentation, and individual program segments became the units exchanged between secondary storage and RAM. A segment was the program's entire code segment or data segment, or sometimes other large data structures. These segments had to be contiguous when resident in RAM, requiring additional computation and movement to remedy fragmentation.

Ferranti's Atlas, and the Atlas Supervisor developed at the University of Manchester (1962), was the first system to implement memory paging. Subsequent early machines, and their operating systems, supporting paging include the IBM M44/44X and its MOS operating system (1964), the SDS 940 and the Berkeley Timesharing System (1966), a modified IBM System/360 Model 40 and the CP-40 operating system (1967), the IBM System/360 Model 67 and operating systems such as TSS/360, CP/CMS and the Michigan Terminal System (MTS) (1967), the RCA 70/46 and the Time Sharing Operating System (TSOS) (1967), the GE 645 and Multics (1969), and the DEC PDP-10 with added BBN-designed paging hardware and the TENEX operating system (1969).

Those machines, and subsequent machines supporting memory paging, use either a set of page address registers or in-memory page tables to allow the processor to operate on arbitrary pages anywhere in RAM as a seemingly contiguous logical address space. These pages became the units exchanged between secondary storage and RAM.