Optical particle counters (OPC) employ a small sensing volume, either by a focused incandescent lamp or by a laser source.In such instruments it is important to avoid coincidence errors resulting from more than one particle in the sensing volume.The instrument manufacturer specifies the maximum number concentration which can be handled. Generally, commercial instruments handle concentrations of up to ~106 particles/litre.Beyond this concentration limit, sample dilution is usually used, which decreases the accuracy of the determination of the concentration.The range of particle diameter that single-particle instruments are capable of handling is ~0.1 μm. Optical particle counters have found wide use, first in cleanroom monitoring and more recently in community air pollution and industrial hygiene studies. A number of laboratory instruments employing single-particle scattering have been constructed.A critical review of such instruments is given by Chigier and Stewart [10].