The technique of near infrared spectroscopy (NIRS) has been widely used for several years as a monitor of tissue oxygenation in intact organs. The aim of the work described in this thesis was to investigate the applicability of the technique in performing quantitative measurements of cerebral haemodynamics in the adult. The physiological background to the project is presented in the first chapter and describes the role of the cerebral circulation and the current methods employed for the measurement of cerebral blood flow (CBF) and cerebral blood volume (CBV) in adults. The physical principles behind the technique of NIRS are then described in the second chapter, with particular emphasis on the behaviour of light within a highly scattering media such as biological tissue. Chapter Three completes the technical background with details of the current instrumentation used for NIRS measurements. Physiological measurements have been performed on healthy adult volunteers for the measurement of CBF, CBV and mean cerebral saturation (SmcO2). These studies are described in the fourth and fifth chapters together with a detailed interpretation of the results. The relatively high blood flow of the adult brain imposes limits on the hardware and software requirements for the measurement of CBF and the resulting methodological problems and sources of error are also discussed. A description of the measurement of changes in cerebral haemodynamics during inspiration and expiration is given to demonstrate the use of the technique in monitoring rapid changes in cerebral oxygenation. Chapter Six describes the development and testing of a novel analysis method for the calculation of CBF. It is hoped that this analytical method, which is based upon the measurement of cerebral vascular transit time, will improve the clinical usefulness of the technique.