Recent advances in focused beam reflectance technology have resulted in new instruments with the potential for particle size determination in natural waters. A commercially manufactured instrument based upon this principle has been adapted to allow the size distribution of suspended particulate matter in estuaries and coastal waters to be determined in-situ. After calibration, the standard instrument has been shown to be capable of providing accurate, undisturbed particle size distribution data over a wide range of particle concentrations (10 mg 1^-1 - 50 g 1^-1 ), irrespective of current speed and, to a large extent, particle composition and density. Following modifications, the instrument underwent field trials and deployments within the Humber Estuary and Coastal zone along the East Coast of England. A further study as part of LISP (UK), upon the Intertidal mudflats of Spurn Bight demonstrates the instruments flexibility and potential in providing high-resolution data. Data obtained from these studies have revealed large temporal and spatial variations in the size distribution of suspended particles over a tidal cycle and along the estuarine salinity gradient. The largest particles occurred near the bed during periods of slack water where median diameters reached 500 µm. This was attributed to the sedimentation of large aggregates. Rapid reductions in median particle size coincided with periods of strong tidal current acceleration after low water slack current. At maximum current velocity on the flood tide, mobilisation of sediment from the bed gave rise to larger particles in the water column. Under similar conditions at low water slack in the turbidity maximum, aggregation of the much smaller floes (90 µm) and subsequent sedimentation did not occur. Particle sizes remained low until high water slack ( 400 µm) when lower particle concentrations were encountered. Multivariate analysis indicated that combinations of current velocity, shear, turbulent eddy scale and particle concentration provide site-specific predictors of particle size. Despite the occurrence of small particles in the turbidity maximum, the pattern of particle size variation with particle concentration and shear stress is consistent with the conceptual model proposed by Dyer (1989).