This book presents a generic process-based crop growth model, GECROS (Genotype-by-Environment interaction on CROp growth Simulator), recently developed in Wageningen. The model uses robust yet simple algorithms to summarize the current knowledge of individual physiological processes and their interactions and feedback mechanisms. It was structured from the basics of whole-crop systems dynamics to embody the physiological causes rather than descriptive algorithms of the emergent consequences. It also attempts to model each process at a consistent level of detail, so that no area is overemphasized and similarly no area is treated in a trivial manner. Main attention has been paid to interactive aspects in crop growth such as photosynthesis-transpiration coupling via stomatal conductance, carbon-nitrogen interaction on leaf area index, functional balance between shoot and root activities, and interplay between source supply and sink demand on reserve formation and remobilization. GECROS combines robust model algorithm, high computational efficiency, and accurate model output with minimum number of input parameters that require periodical destructive sampling to estimate.