This article, a 1997 Best Paper Award winner, has been made available to all net users by Measurement Science and Technology, a journal dedicated to "the theory, practice and application of measurement in physics, chemistry, engineering and the environmental and life sciences from inception to commercial exploitation." The paper (in .pdf and Gzipped PostScript formats) discusses three-dimensional particle tracking velocimetry (3D PTV), "a flow measurement technique for the determination of velocity vectors and trajectories within a three-dimensional observation volume."