In this paper, bond strength and various important factors in bond strength testing are discussed as well as the limitations in the interpretation and clinical relevance of such tests. Standardisation of bond strength testing is needed, and the solutions found in the new ISO document, ISO CD TR 11405 Dental Materials--Guidance on testing of adhesion to tooth structure, are referred to. Tensile and shear test methods are discussed and the bond strength values obtained with these methods compared. The influence of dentine substrate variations, such as remaining dentine thickness and surface treatment or removal of smear layer, are discussed as well as the storage conditions of specimens for in vitro tests, that is, short term, long term and thermocycling, and their relevance to the clinical situation. The information obtained from microscopical studies of fractured surfaces indicates that some adhesives may, under optimal conditions, obtain a bond strength sufficient to fracture the dentine.