We review recent progresses in Resistive Random Access Memory (ReRAM) technologies together with difficult challenges and prospects. ReRAM is one of the most promising emerging nonvolatile memories, in which both electronic and electrochemical effects play important roles in the nonvolatile functionalities. First, a brief historical overview of the research is provided. The technological overview is reported with the epoch-making achievements. Second, the current understanding in terms of the operation mechanism is shown followed by the technical assessment, especially the advantages of ReRAM. Finally, we summarize the challenges facing the ReRAM technology and the prospects.