This paper is concerned with indentation testing of brittle materials. It is shown that a mismatch of elastic constants between the indenter and component being tested has a profound influence on the stress state induced. It is shown that the development of surface flaws into cracks is severely impeded if the indenter is more rigid than the substrate and vice versa. A quantitative assessment of this phenomenon, in terms of both the contact stress field generated and the stress intensity factor experienced by defects, is given. This permits a more precise determination of either the fracture toughness or the surface flaw distribution to be made.