An important part of any groundwater modeling exercise is the model calibration process. In order for a groundwater model to be used in any type of predictive role, it must be demonstrated that the model can successfully simulate observed aquifer behavior. Calibration is a process wherein certain parameters of the model, such as recharge and hydraulic conductivity, are altered in a systematic fashion and the model is repeatedly run until the computed solution matches field-observed values within an acceptable level of accuracy.The model calibration exercise in this tutorial is based on the MODFLOW model. Thus, it is recommended to complete the “MODFLOW – Conceptual Model Approach I” tutorial prior to beginning this tutorial. Although this particular exercise is based on MODFLOW, the calibration tools in GMS are designed in a general-purpose fashion and can be used with any model.