A laser technique for simultaneous determination of particle size and velocity is proposed and the requisite theoretical analysis presented. The method utilizes light scattering or fluorescence signatures from individual particles passing through parallel laser beams to specify completely the particle trajectory, including two velocity components and the traverse position, in the plane perpendicular to the laser beams. The incident-laser-intensity history of the particle can then be calculated, which allows the particle size to be unambiguously determined from absolute light-scattering or fluorescence measurements.