Photoallergic contact dermatitis (PACD) is a clinical problem that has often been poorly understood and neglected by dermatologists over recent years. This can be partly attributed to its investigation by photopatch testing (PPT) falling between the expertise of photobiologists and contact dermatitis clinicians. One result of this situation was that no European Baseline PPT series had been agreed on. To redress this, the European multi centre photopatch test study (EMCPPTS) was conceived to provide up to date information on which photoallergens are of greatest clinical relevance. Its conduct and results form the core research project of this thesis. To enable the EMCPPTS to proceed and its results to be viewed in a wider context, the other Chapters of this thesis explore important related aspects of PACD and PPT in Europe. The introduction examines the nature of PACD and PPT and reviews current photoallergens. Then, the investigation of the two photoallergens carprofen and chlorproethazine by PPT is recounted. These studies highlight deficiencies within the current European regulatory system for preventing photoallergens from reaching the marketplace, as well as providing templates for the investigation of new photoallergens in the human environment. This is followed by a pilot PPT study which provides new information on the optimum non-irritating concentration of the 19 ultraviolet sunscreen absorbers to be used in the EMCPPTS. The issue of attempting to determine the photoallergenic potential of the EMCPPTS agents with respect to exposure patterns is addressed by conducting a sunscreen survey in the UK. The EMCPPTS itself is then detailed, as well as the difficulties that can be encountered when conducting a large clinical study of this nature. The results from the EMCPPTS and other presented studies were shown to be of importance in deciding upon a new European Baseline PPT series. The process involved in deciding this series, as well as its content are described before overall conclusions and possible future studies in the domain of PACD and PPT are discussed.