Knowledge and understanding of particle size data is crucial in a wide range of industries, being vital for the pharmaceutical industry, with applications from drug development to production and quality control. This chapter introduces the problem of particle sizing in the domain of the pharmaceutical industry. It addresses the basic concepts associated with particle sizing, providing a basis to understand the most important details associated with particle sizing data and its interpretation. The chapter describes the light microscopy‐based methods and the incontrovertible light‐scattering methods. The ability to determine the aerodynamic particle size is of particular importance, for instance to characterize orally inhaled dosage forms, as the knowledge of this parameter is quite useful for predicting the regional deposition of the particles in the human respiratory tract. Cascade impactors are based on inertial separation and allow direct measurement of mass‐weighted aerodynamic particle size of drug substance in aerosols.