This tutorial discusses in depth the operational validation of simulation models after a brief overview of verification and validation of simulation models. The discussion of operational validation first covers the different approaches used for observable and non-observable systems. Next, various types of graphical displays of model output behavior are presented; this is followed by how these displays can be used in determining model validity by the model developers, subject matter experts, and others when no system data are available; and how these displays can be used as reference distributions for operational validation when system data are available. Lastly, the use of the “interval hypothesis test” is covered for operational validation when sufficient system data are available. Various examples are presented.