Redox-flow battery systems have been investigated for sixteen years. Initially developed in the U.S.A., such batteries have since interested many researchers worldwide. It seems that development is not yet exhausted and, in recent years, many industries (practically all in Japan) have made great efforts to produce practical cells. In this paper, following an initial brief summary of the redox principle, the main reports and the activity they describe have been listed, with one or two minor exceptions, historically.