Memristors are nanoscale resistive switching devices. Memristors have been of huge interest for memory, logic and neuromorphic applications in the recent times. Generally, their switching effects in dielectric-based devices are assumed to be caused by conducting filament formation across the electrodes. But the nature of the filaments, their growth mechanisms and dynamics are in huge debate, which demand in situ high spatial resolution characterization techniques. In situ transmission electron microscopy with its imaging, structural and compositional analysis at the nanoscale is an optimum technique to understand the growth mechanisms and dynamics. Through systematic ex situ and in situ TEM studies on nanoscale devices under various programming conditions, the underlying mechanisms can be identified. The results obtained through deserve particular attention for continued device optimization.