Compared to a traditional turbine (Kaplan, Francis, Pelton), pumps operated as turbine are a good alternative for micro hydroelectric power plants thanks to their low cost and easy availability.This paper presents a review of the most challenging aspects of the pump as turbine: prediction and improvement in turbine mode.First, an explanation of the theory of the pump as turbine focused on the functioning, types of pumps, selection criteria and performance predictions as shown in the literature, giving also methodologies and details on the pump as turbine selection.Afterwards, the literature shows the instabilities produced by the operation of the pumps on inverse mode, which generates S-shape curves and cavitation.Finally, as the main focus of this paper, aspects of efficiency produced by geometric parameters were discussed.Current trends and future scope for further improvement and implementation of pump as turbine are also discussed.