This International Standard specifies procedures for extracting fluid samples from a hydraulic fluid power system under operation. The preferred method is to extract fluid samples from a main flowline of an operating hydraulic system in such a manner that the particulate contaminant in the sample is representative of the fluid flowing at the point of sampling. An alternative method is to extract a sample from the reservoir of an operating hydraulic system, but this method should only be used if a suitable sampler is not fitted. The samples taken are used for particulate contamination analysis.