This thesis aims to clarify several issues within the field of direct composite resin materials. A new classification system for composite materials has been suggested, on the basis of the two main components (fillers and the matrix). The mechanical performance of the material when subjected to an artificial fatigue load, simulating that of the oral cavity, has been assessed. An evaluation of colour compatibility, colour stability and contrast ratio of different resin composites and gloss retention and staining behaviour of these materials in response to normal conditions such as exposure to common foods, beverages and brushing habits, was then evaluated. The second part of this thesis is based on the optical characterisation of natural teeth, with particular emphasis on enamel and the enamel-dentine complex. An in-vivo evaluation using a spectrophotometer is proposed for measuring the mismatch between a composite restoration and the corresponding area of the contralateral sound tooth.