This biography reveals the little-known story of medically qualified research physiologist and biochemist, John Macleod (1876-1935). From child of a Free Kirk minister in rural Perthshire, it records his education at the Grammar School and University in Aberdeen. Rapid career progress in Leipzig, Aberdeen and London, culminated in his appointment as Professor of Physiology in Cleveland, Ohio aged only 27. He moved on to similar positions in Toronto then back in Aberdeen. In Cleveland, he became an expert on carbohydrate physiology; in Toronto in 1922, he led the four-man team that discovered the world’s first clinically useful insulin. The following year, he was jointly awarded the Nobel Prize in Medicine or Physiology for this life-saving scientific breakthrough, together with colleague, Frederick Banting. The latter unfairly claimed that Macleod had stolen credit for the work. With important political backing, Banting’s view prevailed sullying Macleod’s reputation and legacy. In a 1982 book, The Discovery of Insulin, Canadian historian Michael Bliss gave a new, detailed interpretation of what had happened in Macleod’s department, concluding that history had underestimated Macleod’s magnificent contributions. Michael Williams (1931-2022), a diabetes specialist, educated at the same school and medical school as Macleod, went on to produce this detailed account of Macleod, originally published in 1993. This refreshed edition, JJR Macleod: Co-discoverer of Insulin, adds descriptions of several interventions over recent years to restore Macleod’s reputation as a scientist whose work has contributed to saving hundreds of millions of lives.