The objectives of this research are to contribute to a better understanding of human platelet involvement in vascular disease. To accomplish this, both the laser light scattering technique and the population balance equation are employed to study platelet aggregation kinetics. Platelet aggregation and related reactions are studied in the uniform, known shear stress field of a rotational viscometer. Laser light scattering gives direct information on the particle size distribution as the reactions occur. The light scattering approach has several advantages over conventional turbidimetric methods. Platelet kinetics results are interpreted by means of the population balance equation (PBE). The PBE mathematically expresses the conservation of mass for a collection of particles interacting with each other. Specifically, the PBE accounts for the formation and disappearance of platelets and platelet aggregates due to collision binding and breakage. The platelet number density distribution is determined both initially and at some later time. Then the PBE can satisfactorily match the observed particle histogram to give kinetic parameters as functions of time, platelet concentrations and magnitude of physical or chemical stimuli. These parameters provide information on adhesive forces and the rates of aggregation and disaggregation of the platelets, and platelet aggregates. These parameters infer the physical properties of platelets and platelet aggregates that are of potential value in increasing our understanding of the processes involved in thrombotic disease and/or therapy.