Abstract : THE STRENGTH OF IONIC SOLIDS IS DISCUSSED IN TERMS OF THEIR RESISTANCE TO FRACTURE. PARTICULAR EMPHASIS IS GIVEN TO THEIR EXTRAORDINARY NOTCH SENSITIVITY. IONIC SINGLE CRYSTALS CAN EXHIBIT CONSIDERABLE DUCTILITY BEFORE FRACTURE PROVIDING THE STRAIN IS LOW ENOUGH AND THE TEMPERATURE HIGH ENOUGH. HOWEVER, THE MERE EXHIBITION OF DUCTILITY CAN BE MISLEADING FOR SUCH CRYSTALS MAY REMAIN BRITTLE IN THE PRESENCE OF A NOTCH. NOTCHES CAN BE FORMED IN A VARIETY OF WAYS; BY MECHANICAL CONTACT, BY THE INTERACTION OF SLIP DISLOCATIONS WITH COHERENT SURFACE DEPOSITS, WITH OTHER SLIP DISLOCATIONS OR WITH GRAIN BOUNDARIES. ONLY WHEN SUBSTANTIAL PLASTIC FLOW OCCURS AT THE ROOT OF THE NOTCH CAN CATASTROPHIC CLEAVAGE BE PREVENTED. THE AMOUNT OF PLASTIC FLOW DEPENDS UPON THE FUNDAMENTAL PLASTICITY OF THE SOLID, THE TEMPERATURE, AND THE STRESS STATE AT THE CRACK TIP. GRAIN BOUNDARIES INVARIABLY EMBRITTLE IONIC SOLIDS IN THE TEMPERATURE RANGE IN WHICH A SINGLE CRYSTAL IS NOTCH SENSITIVE. THIS IS DUE TO THE INHERENTLY LOW COHESIVE STRENGTH OF A GRAIN BOUNDARY, WHICH RUPTURES TO FORM A NOTCH(THE SIZE OF WHICH DEPENDS ON THE GRAIN SIZE) UNDER THE CONSTRAINT STRESSES DEVELOPED BY LIMITED PLASTIC DEFORMATION. AT TEMPERATURES IN EXCESS OF 0.5 TM GRAIN BOUNDARIES HAVE EXTREMELY LOW SHEAR STRENGTH CAUSING POLYCRYSTALLINE MATERIAL TO FAIL EVEN AT HIGH STRAIN RATES.