This article presents a critical review of the studies performed to investigate combustion-chamber crevices as sources of hydrocarbon (HC) emissions, and an evaluation of potential technologies for reducing these emissions. Of the combustion-chamber crevices, the piston upper crevice volume is the main contributor to engine-out HC emissions. Chamfering the piston crown or reducing the top-land height and/or the volume behind the top compression ring may result in significant reductions in HC emissions. Modest reductions may also be achieved by reducing the central-electrode crevice of the spark-plug. However, reducing the head gasket crevice of current production engines appears to have little effect on engine-out HC emissions. In general, the sensitivity of the HC emissions to the combustion-chamber crevices is influenced strongly by the in-cylinder flow field and combustion, which influence the concentration of burned gases in the crevice gases.