Particle counters are necessary in semiconductor manufacturing to control contamination and ultimately to achieve high functional yields. Because small particles are so prevalent and their sources so numerous in integrated circuit (IC) processing, special instrumentation and procedures are needed to permit accurate diagnoses of particle-caused impacts on yield. Particle-counting instruments are sophisticated in design and sometimes appear to be temperamental in response. Calibration of particle counters for particle size thresholds has an added complexity when using instruments designed for counting particles on wafer surfaces. The calibration of a particle counter can be meaningless if errors occur in the calibration process. This chapter categorizes these potential error sources as: particle related (due to the particles themselves or the medium that surrounds them); sample related (due to the means of collecting the particles to be counted); optics related (due to the optics of the instrument); and processing related (due to the electronics circuitry and internal data processing).