The quest for increased memory requirements of the applications over the past few decades have compelled the Operating System designers to come up with products that support virtual memories that in turn can provide the required support for the execution of programs exceeding the size of available main memory.  This has been achieved by virtual memory systems and Demand Paging. This article has provided a detailed overview of the concepts of Virtual Memory and Demand Paging.  It has also highlighted on the merits and demerits of Virtual Memory systems.  These are concepts that I feel should be very clear to any IT professional irrespective of the technology that he or she is working on.  I hope that the readers will find this article a useful resource for understanding these concepts in depth.  I would welcome readers' comments and suggestions.  Happy reading!