A laboratory often is faced with the task of calibrating particle inertial classifiers or investigating how instruments, or systems, respond to particles of specific sizes. Since, over time, multiple technicians may be involved in the calibrations, it is imperative that the procedures be uniform. For this purpose, "Good Laboratory Practice" (GLP) procedures as outlined in Code of Federal Regulations (CFR), Volume 21, Part 58 can be followed. There are three main features of GLP that can be employed in calibrations with particles. They are: 1) all equipment that are used for measuring flow rates, pressure, volume, etc. are calibrated to NIST traceable standards, 2) a calibration protocol is written, and 3) Standard Operating Procedures (SOPs) are defined for all steps of the calibration process. The SOPs include management structure, document control, data handling, procedures for training personnel, calibration of the equipment, and the actual calibration for the device in question. Although these GLP procedures are applied to nearly all calibrations performed in our laboratory, this paper will emphasize procedures suitable for the calibration of inertial classifiers, specifically cascade impactors.