He died peacefully at his home in London, in the arms of his beloved wife Anna, at the age of 91. Jack enjoyed a long and colorful professional career. Perhaps best known for his contributions to "fractionation" in radiation oncology, Jack was a prolific thinker, writer, and speaker over many decades. His academic career was launched strongly in physics at the University of London (BSc 1944, MSc 1946, PhD 1955) (Fig. 1) with contributions in radiation dosimetry and dose-rate effects but also in particle radiobiology and reduced oxygen effects of fast neutron radiation (he worked at the Medical Research Council Cyclotron Unit). However, Jack was intrigued by radiation biology early in his career and commenced tissue and animal experimentation on fractionation effects in normal tissues. Indeed, Jack earned his DSc 1974 in radiation biology and by mid-career was highly engaged in cancer biology, with subsequent interest in radiation fractionation schedules, tumor proliferation, and clinical outcomes. Jack served as director of the Gray Laboratory in Northwood, United Kingdom from 1969 to 1988, following Hal Gray and Oliver Scott in this leadership role. During Jack's tenure, the Gray Laboratory boasted a robust cadre of scientific talent, including Ged Adams,