Abstract: Resisitive random access memories (RRAMs) are one of the most promising next-generation non-volatile memory devices, based on reversible switching between high and low resistance states by the application of an external electric field. They have been widely studied as a remarkable new type of memory device, due to their potential for scaling down beyond the 32nm node limit to replace current mainstream flash memory devices. However, controversy about the resistance switching mechanism of RRAMs has severely limited their further development and application. In this article certain essential models of the charge-transportation in the bulk material are described, and present theories explaining the resistance switching mechanism are also reviewed.