All air particle counters that detect and classify particles using laser scattering are calibrated according to ISO 21501-4. The calibration should take place annually. However, most users do not know exactly what this calibration entails, what the statements “calibration passed” or “not passed” mean in detail, or why two identical devices at the same location can still produce different results. This presentation provides an insight into the functioning of a laser light-scattering particle counter and how it is calibrated.