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Wear and erosion involve loss of material. This may be, for example, because of the absence of adequate lubrication, the rubbing together of components that are supposed to have clearance between them, or from the handling of abrasive materials that impinge on the component that continually removes surface material. In many cases, the presence of debris buildup may provide critical understanding of the specific wear/erosion mechanism involved in the failure. A common wear problem that is encountered relates to bearings, either roller‐element bearings or plain bearings. Features to be looked for include the nature of the damage, for example, overall wear or scores in the bearing surfaces. In the latter case, it is particularly important to identify any hard particles that may be embedded and trapped at the ends of the score marks. Where unexpected wear between components in relative motion is encountered, and lubrication is observed to have been adequate, it is important to look for possible contaminants. These may arise from some other component in the system (for example, through the loss of hard chromium plating particles or through the development of corrosion products that are then transmitted through the system in circulating lubricant).