Photoinduced silver cluster formation on AgBr microcrystals is modeled by a nucleation-and-growth process in competition with electron-hole recombination. Nucleation involves the trapping of an electron at a silver atom, followed by the migration of an interstitial silver ion to yield a stable two-atom cluster. The growth stage is the enlargement of the two-atom cluster by additional trapping of electrons and capture of interstitial silver ions to yield the photographically developable latent image. The simulation of silver cluster formation is accomplished with a Monte Carlo procedure which randomizes the sequence of events but weights each event according to a probability determined by the simulation parameters. Events followed are photon absorption, trapping and detrapping of electrons and holes, silver atom formation and decay, nucleation, growth, and recombination. Prior to the simulations, the microcrystal size and shape must be specified, along with the density, depth, and trapping radii of electron traps. Other parameters include the diffusion coefficient of the electron and hole, the recombination radius, the time for capture of interstitial silver ions by trapped electrons, and the lifetime of the silver atom. Simulation over an ensemble of independent microcrystals is done with a transputer-based parallel processor. Exemplary results showing the dependence of silver cluster formation efficiency on trap depth and trap density as produced by chemical pretreatment of the microcrystals are included. For the high-irradiance condition studied, the trends show that as the cluster size increases, the required trap depth and trap density for maximum formation efficiency decrease.