This thesis investigates the role of dispersed phase concentration on the drop deformation and breakup process of oil-in-water emulsions under flow. This work is motivated by two earlier studies: one by Mason & Bibette [4], on concentrated viscoelastic emulsions, which became monodisperse under the application of a simple shear flow, and a second, by Aronson [5], where a similar emulsion became monodisperse in a complex mixing flow. In the first part of the thesis, we study the effect of changing surfactant concentration, dispersed phase concentration and flow rate on the Aronson emulsion and two other oil-in-water emulsions. We find that emulsions in a mixing flow only become monodisperse around the close packing oil concentration of 70%. Increasing mixer speed and surfactant concentration decreases the polydispersity. Final mean drop sizes follow the same trend as the polydispersity. Qualitatively similar results are obtained by subjecting the emulsions to a simple shear flow. In the second part of the thesis, we directly visualize drop breakup in a concentrated emulsion. We used a model emulsion of castor oil in aqueous maltose in a simple shear flow, over an oil concentration range of 2--75%. Above 20% oil concentration, we observe qualitative changes in the deformation process. Instead of deforming into well-defined cylindrical threads, drops deform into convoluted shapes, with fluid accumulating unevenly along the drop length. The end-pinching instability is also suppressed with increasing concentration. The time to breakup for the drops becomes increasingly independent of initial drop size with increasing concentration until at around the close packing volume fraction, all the drops in the emulsion break together.