The Department of Conservation and Technology at the Courtauld Institute of Art offers a three year postgraduate Diploma in the Conservation of Easel paintings. Under supervision, students carry out historical and scientific investigations of paintings which influence their decisions about their subsequent conservation treatments and display. Three case studies have been selected which illustrate the relationship between scientific examination and conservation of the paintings, the interpretation of scientific data and its influence on choice of treatment. Cases were selected which are both typical of the approach to conservation training at the Courtauld Institute and interesting as unique objects which presented a range of different problems, including assessment of condition, appraoch to cleaning and identification of the deterioration of original materials. The paintings chosen for discussion show, in particular, the role of science in the treatment of the works, and the point at which the work of the scientist ends and the work of the conservator begins.