In this article a model is presented to simulate the growth of tropical rainforests. The model is individual based. Trees of the same diameter classare performed in one cohort. The main part of the model is the competitionfor light in plots of 20 m x 20 m in size. A carbon cycle is calculatedincluding processes like photosynthesis and respiration. Due to a treegeometry submodel typical tree variables (e.g. diameter, height, crown length)are calculated. The mortality process is mainly driven by falling trees andthe canopy gaps they are creating. Some simulation results for a primarylowland dipterocarp rain forest in Sabah, Malaysia are discussed. The resultscan be interpreted that rain forests grow in a layer structure. The modelhas the potential to analyse spatial structure of gap formation in rainforests as well as to simulate a huge number of different species groups.The analysis of logging scenarios can estimate the effects of human impactsin tropical rain forests.