Sidney J. Holt (1926-2019) was more than a founding father of quantitative fisheries science, and the man who "helped save the great whales."His accomplishments, over a career spanning seven decades, run deeper: he was a champion of reductionism (i.e. able to identify the factors essential for management) and a systemic thinker who inspired scientists to think critically about marine conservation and management. This article draws on first-hand experiences with Sidney over the last 15 years, when he regularly collaborated with scholars of the ICES Working Group on the History of Fish and Fisheries and the Oceans Past Initiative. Four main themes emerged from our reflections on Sidney's life and legacy, which constitute ongoing scientific challenges: (1) the suitability of maximum sustainable yield as a target reference point for fisheries management; (2) the future of marine mammal conservation; (3) successful implementation of ecosystem-based marine management; and (4) the value of historical perspectives for conservation and management. We consider Sidney's work across these themes, in which he readily collaborated, focused on evidence-based solutions, and, where evidence was lacking, he advocated for the "precautionary principle."We posit there is much that we, and future generations of scientists, can learn from his example.