Technical Abstract: Model calibration, validation and sensitivity analysis are essential processes in evaluation and application of computer simulation models. To date, there is a little information available on models calibration, validation through comparison to experimental data and on their sensitivity analyses to input parameters. This chapter discusses calibration, validation and sensitivity analyses processes of the LEACHM model and its performance using field measured data. The LEACHM model was evaluated for its ability to predict annual water drainage flux and NO3-N leaching losses. Drainage water and NO3-N losses were collected using zero-tension pan lysimeters placed 1.2 m below the soil surface under continuous corn cropping system. LEACHM calibration was performed in terms of the simulative ability of the model to approximate the measured field data collected from control, fertilized and manure treatments. The model was calibrated to the field site conditions using 1989-1990 data from a long term leaching experiment conducted on a Hagerstown silt loam soil. The initial stage of calibration focused on small changes to soil water flow parameters in the model. Calibration then focused on input parameters controlling soil N transformation processes and rate constants in the model. After the calibration process was concluded, the model was validated using 1988–89, 1990–91, 1991–92, and 1992–93 data by comparing model simulations with field measured data. Sensitivity analyses were carried out on the calibrated LEACHM model by changing the values of input parameters within acceptable ranges. The LEACHM model was slightly sensitive to most input parameters; however, it was very sensitive to denitrification rates, initial soil water potential/water content and b-parameter in the Campbell’s equation.