Several previous Hatfield lectures have treated aspects of steel quality, focusing on topics such as microstructures, inclusions, impurity elements, tensile properties and mechanisms of fracture. Other lectures have described the use of steel in engineering applications. In this year's lecture, I discuss developments over the past 50 years that have enabled the quality of steel to be assessed in a quantitative manner. I discuss the role of strength and safety factors in engineering design and consider the benefits and shortcomings of notched impact testing. I then describe the application of analytical assessments based on the concepts of fracture mechanics. These concepts lead to a heightened perception of the critical interactions between levels of service loading, material properties and inherent defect size. Importantly, these interactions can be quantified. Fracture mechanics underpins not only macroscopic engineering structural integrity assessments, but also micro-mechanical models of deformation and fracture, so that it is possible to assess in a quantitative manner the degree of control needed in material processing and fabrication; that needed to generate a desired microstructure, with uniformity and reproducibility; and that needed to specify and control steel chemistry, inclusion content and level of trace impurity elements.