Particulate contamination in a hydraulic system must be measured to monitor and take corrective action to maintain the desired cleanliness level. Counting methods are reviewed, and the use of Automatic Particle Counters (APCs) is presented as the preferred method. The importance of the calibration method for APCs is stressed, and comparisons of the procedures by American National Standards Institute (ANSI), American Society for Testing and Materials (ASTM), and Society of Automotive Engineers (SAE) are presented. The procedures of ARP 1192A are presented in detail as the microbead technique for calibration. Patents developed to aid in carrying out the procedures are cited. Actions being taken to confirm the procedures of ARP 1192A are presented.