This chapter provides a background on particle sizing and its relevance to the development of emulsion-based drug delivery systems in general. One of the prime physical characteristics of multiphase systems such as lipid emulsions is the size and shape of the particle distribution. Turbidimetry is the measurement of light loss due to its blockage by the presence of a population of particles. One of the most useful techniques available for the characterization of submicron emulsions is single-particle optical sensing (SPOS). SPOS can be implemented in one of two ways, with the differences between them defined by the means with which the light-particle interaction is detected: blockage or scattering. Another approach to SPOS is to take advantage of the light scattering taking place when the particle passes through the light beam in the sensor. The chapter concludes with the demonstration of a multi-method approach applied to formulation problems in the authors’ laboratory.