As a result of the trend towards more difficult operating environments, there is an increasing demand for improved ESP performance and reliability under a range of flow conditions. This poses significant challenges for pump designers, however stage design can benefit greatly from improved understanding, and consequently modelling, of the internal flow behaviour. To some extent this has been successfully achieved, at least for steady, single-phase conditions, through the use of commercial Computational Fluid Dynamics (CFD) packages. However, the complexity of the flows in realistic, unsteady conditions means that more advanced CFD and experimental tools are required for accurate flow characterisation. Here, we will demonstrate how laboratory-based, optical measurement techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) provide new insight into flow behaviour under a range of operating conditions. We also present examples of highly resolved, quantitative velocity fields acquired within impeller and diffuser passages. This information provides improved understanding of the fluid mechanics and can be used directly for design purposes or for validation of CFD models. In addition, unsteady flow computations performed using in-house Large Eddy Simulation (LES), adaptive meshing and immersed boundary methods will be presented. Finally, we will summarise the potential benefits of these new methodologies in the context of pump design.