We describe a set of image processing algorithms for extracting quantitative data from digitized video microscope images of colloidal suspensions. In a typical application, these direct imaging techniques can locate submicrometer spheres to within 10 nn in the focal plane and 150 nn in depth. Combining information from a sequence of video images into single-particle trajectories makes possible measurements of quantities of fundamental and practical interest such as diffusion coefficients and pair-wise interaction potentials, The measurements we describe in detail combine the outstanding resolution of digital imaging with video-synchronized optical trapping to obtain highly accurate and reproducible results very rapidly.