Gorm Greisen and the transformation of our understanding of newborn brain physiology: a tribute on the occasion of his retirement, on behalf of the ESPR NIRS Special Interest Group.

This INSIGHT contribution honours Professor Gorm Greisen on the occasion of his retirement and recognises his profound influence on neonatal brain physiology and care, on behalf of the Near-Infrared Spectroscopy (NIRS) Special Interest Group of the European Society for Paediatric Research. Over more than four decades, Greisen reshaped our understanding of cerebral hemodynamics and the distinctive vulnerabilities of the newborn brain during the transitional period after birth. Through methodological innovation combined with careful physiological enquiry, he contributed decisively to establishing non-invasive cerebral monitoring as a scientifically robust clinical tool and clarified the limits of autoregulation and oxygen-metabolic balance in both preterm and term infants. Beyond his scientific achievements, Greisen has played a central role in international collaborative research, helping to define standards for physiologic trials in neonatology and to cultivate a coherent global research community. His enduring commitment to methodological rigour, thoughtful interpretation, and ethical reflection has influenced clinical practice as well as academic culture. His legacy persists in the conceptual frameworks, collaborative networks, and values that continue to guide efforts to safeguard the developing brain.