The usage of protein isolates and concentrates is increasing globally in the food industry. Protein isolates and concentrates are typically produced in a powder form to facilitate their incorporation into commonly consumed food products. Similar to other powders and flours, determining particle size is crucial for product developers. Proper particle size ensures that protein isolates and concentrates can be incorporated into food product without significantly altering system stability, as particle size influences water absorption, product flowability, and distribution. Particle size also influences the handling process of the powders and flours. In this chapter, three main methodologies for determining particle size and distribution are described. The first method is the traditional sieve shaker technique, which uses mechanical shaking and rotation movement to separate particles by their size. The second method is the jet sieve technique, which uses the power of pressurized air and vacuum to break agglomerated particles and sort them by size. The third method described here is the particle size and distribution analysis using imaging systems. This approach uses high-speed, high-definition cameras to capture the images of the particles as they are purged with pressurized air. This chapter details step-by-step guide to determining the particle size and distribution of protein -rich powders.