Near-infrared tissue spectroscopy is advancing toward clinical use in many areas of medicine. The study of the brain both in terms of hemoximetry and neurophysiological signals is the primary focus of much of the current research in this area. Our laboratory has pioneered the development of a near-infrared tissue spectrometer for clinical use based on the physical understanding of how light travels in tissues. This physical model assumes a homogenous infinite tissue bounded by a flat plane. I propose to investigate two of the fundamental problems involved with application of this model to near-infrared spectroscopy of the brain: the problem of the curved surface of the skull, and the problem of light piping by the cerebrospinal fluid (CSF). Though these problems are present in both adults and infants I will focus on them in the neonatal case. Neonatal brain hemoximetry is of interest because of the correlation of long term pathology including cerebral palsy, attention deficit disorder and mental retardation, with ischemic and hemodynamic insults to neonates. I will investigate the accuracy of a simple model for curved surfaces through in vitro laboratory experiments on tissue simulating phantoms. I will also explore, through Monte Carlo modeling and experimental studies on in vitro laboratory samples, the degree to which the CSF is expected to affect the measurement protocols currently in place.