Sir Brian Keith Follett FRS DL (born 22 February 1939) is a British biologist, academic administrator, and policy maker. His research focused upon how the environment, particularly how the annual change in day-length (photoperiod) controls breeding in birds and mammals and his teaching on physiological adaptations of vertebrates to the environment. Knighted in 1992, he has been a Fellow of the Royal Society since 1984. He was Professor and Head of Zoology at the University of Bristol from 1989 until 1992. Then Vice-Chancellor of the University of Warwick until 2001. Subsequently, he chaired the UK government's Training and Development Agency for Schools and Arts and Humanities Research Council. Until 2019 he was a non-stipendiary visiting Professor in the Department of Zoology (now Biology) at the University of Oxford.

Follett was educated at Bournemouth School and studied biological chemistry. On graduating he undertook a Ph.D. with Professor Hans Heller in the Department of Pharmacology at the University of Bristol. That work introduced him to endocrinology and the development of assays to understand the physiological role of hormones.

In 1964 Follett moved to Washington State University and joined Donald Farner's group investigating photoperiodism. His research focused on the brain pathways whereby birds (and mammals) measure day length and use these changes to change gonadotrophin secretion from the pituitary gland and so regulate breeding. He became a lecturer at Leeds University then moved with James Dodd FRS to the University of Bangor in 1969, then to the University of Bristol as Professor of Zoology and Head of Deparrment in 1978.^{[citation needed]}

He moved to Warwick in 1993 as Vice-Chancellor.

Follett's studies used, as model species, the Japanese quail and later wild-caught starlings. His work included the development of the first radioimmunoassay to measure bird luteinizing hormone (LH) in collaboration with Frank Cunningham (Reading University) and Colin Scanes. This made it possible to measure LH in a few microlitres of plasma and so follow circulating hormone levels in individual birds exposed to photoperiods of many types. This enabled the first detailed analyses of the annual pattern of hormone secretion as well as allowing deeper questions to be asked: for example, using gonadectomized quail to show unequivocally that the underlying photoperiodic response in birds (but not mammals) is driven by brain circuits that are switched on and off by day length and to demonstrate that measuring day length involved a daily (circadian) rhythm in photosensitivity with the birds being responsive to light particularly between 12 and 18 hours after dawn. In other words, if light fell at these hours then the day was read as "long", if not then it was read as "short".

In 1978 at Bristol, his research interests expanded to include mammals, notably sheep, and wild birds such as albatrosses, swans, gulls and partridges. Key studies included developing a rapid photoperiodic response system which enabled the detailed study of the neural pathways as they are switched on, the complex structure of the time-measuring system and the action spectrum for the non-retinal light receptors (with Russell Foster). In Bristol the focus also swung towards the mechanisms involved in ending seasonal breeding - so-called refractoriness. The photoperiodic response involves both the induction of breeding and it ending each year. The Bristol group found, quite counterintuitively, that thyroid hormones are critical for refractoriness to develop and be maintained. This had been tentatively suggested in the Soviet Union prior to WWII but was developed by Trevor Nicholls, Arthur Goldsmith and Alistair Dawson. Importantly birds are hatched in a refractory state but this is ended by removing the thyroid glands (per Tony Williams). The research group's papers on this have proved of general value in understanding the seasonal control of the neuroendocrine pathways in not only birds but also in mammals.

Funding came from the Agricultural and Food Research Council (AFRC), later renamed the Biotechnology and Biological Sciences Research Council (BBSRC), and Follett's group became an official Research Council Research Group on Photoperiodism and Reproduction, with 413 scientific papers and reviews.

Follett was head of the department of zoology (later biological sciences) at the University of Bristol for fifteen years (1978–1993), and Biological Secretary of the Royal Society from 1978 until 1993. He was then vice-chancellor of the University of Warwick for eight years.