Substantial progress has been made in rice growth modeling, and several simulation models, such as CERES-Rice, ORYZA2000, SIMRIW, RCSODS, and RiceGrow, have been developed and tested across diverse conditions. These simulation models quantify the processes of growth and development in rice and their relationships with environmental factors and cultural practices. As a typical modeling approach in rice, nitrogen (N) concentration and protein formation in grain are determined by calculating the fraction of N partitioned into grains based on the final amount of N accumulated by the plant and adjusted by a N harvest index. After the seedling stage when seed N is depleted, N uptake from the soil is required to meet the needs of physiological functioning, structural growth, and subsequent grain protein accumulation after anthesis. N-uptake rates vary among genotypes and environmental conditions. N for the growth of new plant organs is primarily from two sources: uptake by the roots and remobilization from aging organs.