This chapter describes how nodule growth and nitrogen (N) fixation are simulated in the CROPGRO-legume model in relation to N-uptake processes. It introduces the theory of N deficit that causes the increased carbohydrate availability that is used to drive nodule growth and nitrogenase activity. The GLYCIM model runs hourly, with hourly partitioning to organs including seeds. During one of these modeling sessions, Dr. Acock found that the model’s seeds were aborting every night for lack of N. Major code improvements in nodule growth and N fixation were made by K. J. Boote for the V3.1 release of the CROPGRO model, followed by additional minor changes, culminating in the 1998 release of CROPGRO V3.5. Since the CROPGRO model simulates a number of different grain legumes, there is a similar problem of how to estimate the temperature sensitivities and other parameters for the various legumes in the absence of data.