The dermal route is an important component of exposure to occupational and environmental chemicals. Skin (stratum corneum) acts as a barrier to the passage of chemicals into the body and to predict systemic risk and set safety standards reliable measures of rates of percutaneous penetration are required. Useful data can be obtained from in vivo human volunteer and animal studies, dermal occupational biomonitoring, predictions from structure activity relationships and PBPK models and increasingly percutaneous penetration measurements with isolated human or animal skin maintained in an in vitro diffusion system. Studies with animals can be related to available toxicology data whereas human skin is most relevant for human risk assessment. It is important to standardise in vitro penetration studies and to validate, with parallel in vivo studies. Many different in vitro systems are in use and a number of organisations have produced guidelines. A standardisation study with 10 European laboratories has recently been conducted as part of an EI funded research project. Studies with a flow through diffusion cell with rat skin and various receptor fluids have been directly compared to in vivo studies in the rat using the same dose, vehicle and application time for a range of chemicals of differing physicochemical properties and lipophilicities. In vitro studies with human skin have been related to human volunteer studies. In vitro experiments can also be designed to reproduce work place scenarios and confounders such as multiple doses, mixed exposures, vehicles, exposure patterns, local skin damage, local metabolism investigated and occupationally relevant finite doses compared to infinite doses. In conclusion, results indicate that the in vitro penetration rate and distribution is influenced by the source of human skin but as with the rat generally reflects passage through the stratum corneum and subsequent distribution in vivo.