Understanding the vegetation dynamics of a grass sward at fine-scale requires breaking the interplay between density-dependent processes and environmental variability. Simulation of one of these interacting processes may be a powerful tool. SISTAL is a spatially explicit, object-oriented model of the spatial distribution and spatio-temporal dynamics of individual grass tillers in cut swards. It focuses both on the spatial movement of existing tillers and on the birth and death of tillers. The time management is based on discrete events, and the system dynamics is computed with thermal time. The space management uses both a spatial grid and real coordinates. The paper exposes implementation choices that have been carefully selected: inputs and outputs; edge effects; implementation of the memory management and the handling of pseudo-parallelism. The simulation diary is described since it was found a very useful tool for verification. Essential results and comparisons with real observations allowing the validation of the model are presented.