The degree of acceptable risk of fracture in engineering components ia discussed in terms of the responsibility which the designer can be celled upon to beer for fractures that occur in service. The common bases of calculation of the dimensions of load-bearing parts are described in terms of the material yield and ultimate strengths, factors of safety and load factors, and the small extent to which these bear upon the incidence of fracture by brittle, fatigue, corrosion and creep cracking is indicated. Experience of these modes of fracture is illustrated in a historical pattern.