Instruments used in biopharmaceutical applications are generally classified as either critical or non-critical, with some also allowing a third classification of diagnostic or reference. Critical instruments are calibrated quarterly as their failure or incorrect calibration could immediately impact product quality, while non-critical instruments are calibrated annually as they may impact secondary systems or non-quality parameters. The definitions of these classifications require some judgment in application and allow for flexibility, as not all systems can be defined exactly. Every instrument must be identified with one of these classifications and have a unique identification that remains with it for scheduling and record keeping.