The correlation of the reflectance of light in water with measurements of water quality has become of increasing interest in recent years as it is seen as a key to the possible interpretation of remotely sensed ocean colour data. Howevert attempts to do this have met with only limited success. There has been a poor level agreement on the values of critical constants used in algorithms, and data collected at sea are characterised by a high level of varianceo The problem has been addressed with particular sub-surface attention to the measurement of the reflectance, R. Theoretical and practical aspects of the measurement of R have been investigated, a number of optical instruments have been built and a programme of in-water optical measurements has been carried out to test and demonstrate hypotheses. Major sources of error have been identified and methods of overcoming them or limiting the magnitude of their influence have been proposed. A set of criteria for the design of a suitable instrument has been formulated, and both a prototype and a full ocean-going version of an instrument based on those criteria have been designed and built. Data collected at sea using both versions of the reflectance measuring instrument have been used to develop a discussion of problems and possibilities associated with the interpretation of sub-surface reflectance data.