One of the most important of all concepts related to Memory Management is Virtual Memory.  Virtual Memory refers to the concept whereby a process with a larger size than available memory can be loaded and executed by loading the process in parts.  The program memory is divided into pages and the available physical memory into frames.  If a process attempts to access a page that is not available in the main memory and the information of which does not exist in its page table, a page fault occurs.  The Operating System now takes care of swapping this page in to the main memory from the backing store.  The Operating System follows its predefined algorithms for page replacement.  Demand Paging refers to loading a page of program code from disk into memory as and when it is required by the program.  The region of the backing store that is used to store the swapped out pages from the main memory is called swap space.  This article discusses Virtual Memory with special reference to Demand Paging and sheds light on some the most commonly used page replacement strategies that are adopted by the Operating Systems.