**4. Conclusion**

In this review, we have outlined an overview of memristor-based synaptic devices, especially for the metal oxide memristors. The neuromorphic approach with oxide-based RRAM devices is promising. Focusing on TiOx , WOx-based memristor, the electrical switching characteristics are reviewed. Exploiting the physical mechanisms, the synaptic behaviors of those devices are also discussed. Owing to the magnificent increased computational efficiency, and also increasing compatibility in computer technology and CMOS technology, metal oxide-based synaptic devices are gaining prominent interest. The progress of neuromorphic engineering on devices confirms that the memristive synapses can meet the demand of low energy consumption, high connectivity, and density in neuromorphic devices for efficiently encoding, storing, and processing information. However, challenges still remain for overall oxide-based RRAM materials. Although the inherent fault tolerance of neural network models is able to mitigate the impact of device variation to some extent, the improvement of spatial variation and temporal variation turns out to be one of the greatest challenges on a long-term basis. In addition, the improvement of reliability characteristics of the memristor synaptic devices is another key challenge which is not well studied.
