Calibrating a hydrologic model is the process of modifying parameters within acceptable ranges to obtain simulated results that replicate known conditions. Model calibration is necessary to provide some level of confidence that the simulated results adequately represent the modeled system. To calibrate a model, observed data, typically flow discharges and stages collected in the field, are compared to the simulated results. This chapter introduces the concept of calibration, describes the procedure used in manual calibration and discusses some of the summary statistics used to evaluate the calibrated model. Automated calibration procedures are discussed in the Optimization chapter.