Particles are ubiquitous in all natural systems and play an important role in the control and fate of nutrients and pollutants. Submicron particles are especially important because they have large specific surface areas and high surface free energies which facilitate sorption of significant quantities of substances. Currently, only limited information is available on particle number and size distributions of natural submicron particles, owing to the significant problems involved in their experimental determination. In the present paper, existing data are discussed and compared with theoretical predictions based on a classical coagulation/sedimentation model where the kinetics of coagulation is described by Smoluchowski's equations and sedimentation is characterized by Stokes' law. Model predictions agree well with reported particle behaviour for particles larger than 100 nm. While classical theory does not explain the presence of colloids smaller than 100 nm in surface waters, the existence of such small colloids, embedded in large organic matrices, has been observed recently by high resolution transmission electron microscopy. Improvements in the measurement techniques available as well as development of the necessary theory to describe these organic/inorganic associations would be most welcome.