We report near-optimal tracking of freely diffusing fluorescent particles in a quasi-two-dimensional geometry via photon counting and real-time feedback. We present a quantitative statistical model of our feedback network and find excellent agreement with the experiment. We monitor the motion of a single fluorescent particle with a sensitivity of $15\mathrm{nm}∕\sqrt{\mathrm{Hz}}$ while collecting fewer than $5000\text{fluorescence}\text{photons}∕\mathrm{s}$ . Fluorescent microspheres (diffusion coefficient $1.3\mu {\mathrm{m}}^2∕\mathrm{s}$ ) are tracked with a root-mean-square tracking error of $170\mathrm{nm}$ , within a factor of 2 of the theoretical limit set by photon counting shot noise.