The findings of a long-term investigation of the wear of cylinder liners are discussed. The actual liners examined were drawn from a wide range of those commented on by customers in the field. The main objective is to present evidence about the wear mechanisms that appear to be associated with a whole spectrum of behaviour reported by engine operators. Scuffing is generally accepted as the main reason for high wear rates, and a likely explanation for this is that as power outputs have increased, it has become increasingly difficult to maintain an adequate oil film between the piston and liner surfaces. Under conditions of marginal lubrication, metal-to-metal contact occurs and the metallurgy of the surface becomes important. Order from BSRA as No. 49,822.