During a distinguished career, John Wyrill (‘Jack') Christian had a profound impact on the subject of materials science, particularly physical metallurgy. He was recognized as a world authority on martensitic transformations, and laid the foundations forthe modern understanding of this topic. His monumental two–volume work, The theory of phase transformations in metals and alloys, is the classic authoritative treatise on the subject, and remains one of the most important texts ever published in the area of materials science. It redefined the whole field of phase transformations, set new standards of intellectual rigour and comprehensiveness, and inspired successive generations ofscientists to follow in his footsteps. He was also a pioneer in the study of the mechanical properties of metals and alloys, particularly those having the body–centred cubic structure. He and his students played a key role in establishing that the low–temperature mechanical properties ofthis important class of metals are controlled by intrinsic dislocation–lattice interactions and not by impurity effects. He contributed tothe study of many other topics in materials science, including the structure of interfaces, the mechanism of deformation twinning, and the properties of stacking faults. He was the recipient of numerous national and international awards for his work. His researches, which were always characterized by precision and deep physical insight, have stood the test of time.