The redox flow battery (RFB) is considered to be a promising large-scale energy storage technology due to its unique advantages, including long lifetime, simple structure, and decoupled power and energy. However, RFB's inherent drawback of relatively low energy density needs to be addressed. To further improve their performance and efficiency, many novel RFB systems have been developed. In this chapter, the basis of RFB, including the working principle, configuration, and important parameters to evaluate the performance of the battery, is discusses. The properties and performance of conventional aqueous and nonaqueous RFBs are discussed. Two types of new flow battery systems in terms of semi-solid and redox-targeting RFB are covered. The properties of two important components of RFB, membranes and electrodes, are also discussed.