Calibration is the process of evaluating preliminary model output and adjusting parameters to get a more realistic result. Calibration performs many important tasks—the user (1) develops a solid feel for model behavior from the iterative runs, (2) becomes more confident in interpreting results, (3) detects obvious problems with initialization and parameterization, (4) learns the sensitivities of model parameters, and (5) prepares the model for the execution of the project. This chapter presents the eight steps of calibration: (1) identify important parameters to use as knobs for calibration, (2) develop a calibration strategy to ensure objectivity, (3) collect any 114data or information useful in the calibration, (4) perform preliminary calibration simulations, (5) compile a final list of parameters, (6) adjust parameters, (7) run model, and (8) evaluate results and repeat. Last, a set of issues that emerge during calibration are addressed and solutions are presented.