This SAE Aerospace Recommended Practice (ARP) provides procedures for assuring cleanliness of sampling equipment, for performing the sampling process, and for measuring and reporting the sample particle count. The ARP gives procedures for cleaning sample bottles, when used, and recommends the solvents to be applied and how these solvents should be prepared. Requirements for the selection of the sampling point, sampling method, and sampling frequency are also specified. This ARP also recommends three measuring methods for determining the level of solid particle contamination of hydraulic fluids used in aerospace hydraulics. These are: a. on-line automatic particle counting; b. automatic particle counting method using bottle samples; and c. microscopic particle count method using bottle samples.