A review of research studies that focus on the validity of portion-size measurement aids (PSMAs) was undertaken because guidelines for the use of PSMAs in studies of food intake have not been developed. Also, the accuracy of food quantity estimates may greatly influence the quality of dietary intake data. Generally, the review indicated that differences between the food quantity estimates obtained from different PSMAs were not statistically significant. Research on PSMA accuracy was lacking in several areas. Additional information is needed on the validity of PSMAs, particularly as it relates to PSMA use by different socio-demographic groups. Little research has focused on the specific effects that PSMAs have on quantity estimates or on subject perceptions of PSMA use. Improved reporting of PSMAs in the literature is also needed. Future research should be directed toward these issues so that PSMA guidelines can be developed. Until there is a more complete research base, it is not possible at this time to establish guidelines for use of PSMAs in clinical practice or in research settings.