Reversible pump-turbines are designed with more emphasis on the pump flow, since the decelerated pump flow separation, recirculation and losses. This exigence, however, often leads in combination with the hydraulic system to stability problems near no load (runaway) in turbine operation. Such an instability is associated with head an flow rate fluctuations in the system. Head and flow rate fluctuation in turn lead to torque fluctuations on the shaft in the starting process of the machine and accordingly a prolonged synchronization process results. This retardation is highly unwanted effect, since demands for faster switching times become more and more important in today’s electricity market. Such system instability is never observed for Francis turbines, machines where design rules of turbines are followed. Normally system stability can be accomplished by simply misaligning (de-synchronizing) a few guide vanes when starting the machines, however this is not a cure at source. The occurrence of system instabilities is closely linked to the slope of the head characteristics near no load operation. If the head-flow curve has a negative slope then an energy transfer to the hydraulic arises exciting oscillations in the system. In order to cure the problem at its source, detailed knowledge of the changes in the flow patterns within the pump turbine and of the different dissipation mechanisms involved to reach the equilibrium at no load operation is essential.