ORYZA2000 is a product of the modelling “School of De Wit” (Bouman et al., 1996 and Van Ittersum et al., 2003). It simulates growth and development of lowland rice in situations of potential production, water limitations, and nitrogen limitations. It is assumed that, in all these production situations, the crop is well protected against diseases, pests, and weeds and that no reductions in yield take place. ORYZA2000 is programmed in Compaq Visual Fortran using the FORTRAN Simulation Environment (FSE) as developed by Van Kraalingen (1995). A detailed explanation of the model and program code is given by Bouman et al. (2001). Below, we summarize the model for potential and nitrogen-limited production (version 2.11, April 2004). The model is freely available, complete with source code, full scientific description, supporting libraries and documents, user guidelines and a self-instructive training course, through internet (www.oryza2000.com or www.knowledgebank.irri.org/oryza2000) or by request to the International Rice Research Institute.