Dynamic multiphase flow behavior inside a mixed flow Electrical Submersible Pump (ESP) has been studied experimentally and theoretically for the first time. The overall objectives of this study are to determine the flow patterns and bubble behavior inside the ESP and to predict the operational conditions that cause surging. An experimental facility has been designed and constructed to enable flow pattern visualization inside the second stage of a real ESP. Special high speed instrumentation was selected to acquire visual flow dynamics and bubble size measurements inside the impeller channel. Results showed formation of a large gas pocket at the impeller intake during surging conditions. The theoretical study includes a mechanistic model and CFD simulations for the prediction of the flow behavior inside the pump. The mechanistic model comprises a one-dimensional force balance to predict occurrence of the stagnant bubbles at the channel intake. The model enables the prediction of the operational envelope of the ESP, namely the transition to surging. Results from the CFD simulations are consistent with the experimental data.