Norman Petch was trained in chemistry at Queen Mary College in London, in metallurgy at Sheffield, and in physics at the Cavendish Laboratory, University of Cambridge. The quantitative nature of his crystallographically-based earliest papers, dealing with the positions of iron and carbon atoms in steel, very probably contributed to the meticulous nature of his follow-on mechanical properties research, initially, into the influence of polycrystal grain size on the plastic yielding and fracturing behavior of iron and steel materials at low temperatures. This work, that was complementary to studies of the yield point behavior of steel by his Cambridge colleague, E.O. Hall, led to the well-established Hall-Petch equation for the stress dependence on reciprocal square root of average grain diameter; perhaps, the most widely quoted equation in materials science. Norman Petch's research, with students and colleagues, led on to predictive and/or interpretive Hall-Petch connected relations for the influence of carbon on the friction stress of steel, hydrogen embrittlement influences on the yield and fracture of steel, the ductile-brittle transition behavior of iron and steel and related materials, the plastic deformation properties of brass and aluminum materials, and the fracturing behavior of ceramic materials and laminated composites.