SimulO is a natural 3D forward Monte Carlo, which implies that photons are followed one at a time from the source to the point where they are absorbed. SimulO is written in Delphi®, a visual Pascal object-oriented language. This language allows to create a “user-friendly” Windows interface, while the object-oriented architecture is the most powerful way to create a code to manage a device described by several optical objects. In SimulO, the user can compose complex optical devices by positioning and sizing any number of elementary objects. Presently three elementary shapes are available: sphere, cube, and cylinder.