Particulate materials are often characterized by parameters of their particle size distribution (PSD). For the purpose of PSD measurement, typically, samples are taken from a process line or a product batch, which are then dispersed and/or diluted before the PSD measurement is executed. Usually, the attention on measurement quality is focused on the measurement technique. This article focuses, in contrast, on the sample preparation, as it is often the dominant source of error in overall PSD determination. The following error sources are discussed: (1) the number of particles (sample size) taken into account; (2) the degree with which samples can represent the product batch in view of (potential) segregation; (3) partial dissolution or agglomeration of particles during dispersion/dilution; and (4) ingression of air bubbles in suspensions and emulsions before measurement. If these error sources are not addressed properly, any of them may cause very poor product characterization.