Lindsay Symon was a professor of neurological surgery at the Institute of Neurology, London University and the National Hospital, Queen Square who made important contributions to neurosurgical management, technique and training, and major advances in cerebrovascular pathophysiology. He was born in Aberdeen, the son of William Lindsay Symon and Isabel Symon, and was educated at Aberdeen Grammar School. He was awarded a scholarship to Aberdeen University to study medicine at the age of 17 and graduated MB ChB a month before his 22nd birthday with many prizes and gold medals, including the Lyon prize as the most distinguished graduate of the year in medicine. After appointments as a house physician and then house surgeon to the professors of medicine and surgery at Aberdeen, he joined the RAMC for his National Service, which he spent mostly in Graz, Austria. In Austria he met and married Pauline Barbara Rowland in 1954, a Liverpool-trained Queen Alexandra’s Royal Army Nursing Corps sister. He returned to Aberdeen as a research fellow in surgery and then as a surgical registrar. A Medical Research Council (MRC) fellowship took him to the MRC division of physiology and pharmacology of the National Institute for Medical Research Laboratories at Mill Hill, where he worked with William Feldberg and pursued his interest in the cerebral circulation, publishing in the *Journal of Physiology* on the leptomeningeal circulation in dogs (‘Observations on the leptomeningeal collateral circulation in dogs’ *J Physiol*. 1960 Nov; 154[1]: 1-14.2). He combined this with an honorary attachment to Valentine Logue’s neurosurgical unit at the Maida Vale Hospital. A Rockefeller Travelling fellowship (from 1961 to 1962) took him to the laboratories of John Sterling Meyer, a pioneer in cerebral blood flow research, at Wayne State University in Detroit, USA. On his return to the UK, Symon joined Logue’s department at Maida Vale as a registrar and first assistant until his appointment to the consultant staff of the National Hospitals in 1965. He succeeded Logue as professor of neurological surgery at the Institute of Neurology, University College London and chairman of the Gough Cooper department of neurological surgery at the National Hospitals in 1978. He also held honorary consultant appointments at St Thomas’ Hospital, Hammersmith Hospital, the Italian Hospital and the Royal National Nose, Throat and Ear Hospital. He was adjunct professor in the department of surgery at Southwestern Medical School in Dallas and a civilian adviser in neurosurgery to the Royal Navy. Symon received numerous medals and rewards from institutes around the world, including the Jamieson medal of the Australasian Neurosurgical Society, the John Hunter medal of the Royal College of Surgeons of England, the K J Zulch prize of the Max Planck Gesellschaft in Germany, and the Otfrid Foerster medal of the Deutsche Gesellschaft für Neurochirurgie. He gave the Herbert Olivecrona Memorial lecture at the Karolinska Institute in Stockholm and the Sir Thomas Willis lecture to the Stroke Council of America. He was an active participant in numerous symposia and conferences around the world and visiting professor or invited lecturer in Canada, Australia, New Zealand, Japan, China, South Korea, United States and many European countries. He was made a CBE in 1994. During the 15th World Federation of Neurological Societies’ world congress in Seoul, South Korea in 2013 he received the Samii medal of honour. Symon was particularly interested in the training of neurosurgeons and was a member of the editorial board of *Advances and Technical Standards in Neurosurgery* from its inception in 1972, becoming its chief editor from 1984 to 1994. He was elected president of the World Federation of Neurosurgical Societies and served from 1989 to 1993. During this time, he established a central office and secretariat for the Federation in Geneva and helped to develop educational schemes and courses worldwide. He was vice president of the European Association of Neurosurgical Societies from 1975 to 1979 and a member of the training committee from 1979 to 1983. He was president of the Harveian Society of London in 1998. Symon was well known and much appreciated by his surgical colleagues and referring physicians for his clinical expertise and technical skill. In 1990 *The Observer* newspaper ran a series on the best in various fields as voted by their contemporaries: the survey of British neurosurgeons voted Symon as the best and most distinguished British neurosurgeon. Of around 500 paper publications, about 200 related to clinical aspects of neurosurgery and to technical issues. He was particularly interested in cranial and spinal vascular malformations and aneurysms, and skull base surgery, publishing on acoustic neuroma, trigeminal neuroma, CSF leaks, jugular paraganglioma, craniopharyngioma, haemangioblastoma and meningioma. Technical advances included the development and intraoperative use of somatosensory evoked potentials, central contraction times, intracranial pressure measurements and in acoustic neuroma surgery, electrocochleography and facial nerve stimulation and electromyography. Around 300 papers came from the Cerebrovascular Research Laboratory, which Symon established at the Institute of Neurology with support from the Medical Research Council. In collaboration with many academic colleagues, the output from this unit was prodigious, often requiring the development of new techniques, such as the hydrogen clearance method for measurement of local blood flow, ion-selective electrodes and microdialysis to measure electrolyte changes, evoked potentials and intracranial pressure monitoring. There is no doubt that the most important and lasting contribution was the concept of progressive thresholds of ischaemia, first published in 1977 in the *Journal of Neurological Sciences* (‘Extracellular potassium activity, evoked potential and tissue blood flow: relationship during progressive ischaemia in baboon cerebral cortex’ *J Neurol Sci* 32: 305-21, 1977). Symon and his colleagues showed that there were two perfusion thresholds with ischaemic injury when blood flow in the cerebral cortex was reduced progressively. Evoked responses were affected at cerebral blood flows below about 20ml/100g/min with electrical failure at around 16 to 18 ml/100g/min; but potassium efflux, indicating sustained cell membrane failure, did not occur until the local blood flow fell to around 8-11 ml/100g/min. These observations showed that in focal ischaemia there were areas with blood flow below that required for electrical activity, but above that needed to maintain cellular integrity; these areas Symon and his colleagues designated ‘the ischaemic penumbra’. In these areas there is a precarious balance between blood supply and metabolic demand, and this is time limited, with a tendency to fail, joining the infarct core where the blood flow is inadequate to sustain cell membrane function from the onset of ischaemia. Restoration of blood flow to the penumbra has the potential to restore electrical function, but time is short, almost four hours or sometimes a bit longer. These important observations are now the basis for the acute treatment of stroke with thrombolysis or catheter extraction of clot. Apart from medicine, his main sporting interest was golf, which he played regularly from an early age. When working in London he played at South Herts Golf Club and in retirement, when he and Pauline moved to Shalbourne in Wiltshire, he played at Tidworth Garrison Golf Club, where he was a life member. He was also a member of Rye Golf Club and the Royal and Ancient Golf Club of St Andrews. For several years he raced dinghies on Grafham Water, Huntingdonshire. He was a freeman of the City of London and a member of the Caledonian Club, serving as secretary of the Caledonian senior members golf club for many years. He was a member of the Royal Archaeological Institute and developed interests in medieval history, the prehistory of Wiltshire and the naval battles of the 20th century. Predeceased by his wife Pauline in 2018, he died on 2 December 2019 at the age of 90 and was survived a son, Frasier, two daughters, Rosemary and Fiona, and five grandchildren.