Petrophysical analysis is a factor in a reservoir study because it provides primary input data for characterization of subsurface formations and evaluation of resources. Common petrophysical input data to a reservoir study include porosity, water saturation, permeability, and mineral or rock volumes. These reservoir variables are typically not directly measured by well-logging tools; instead, they are commonly derived through multiple processes, including acquisition, processing, interpretation, and calibration. As each of these steps involves uncertainty, the resultant petrophysical data will have uncertainty and limitations. Common uncertainties in interpreting the most important petrophysical variables from well logs are discussed in this chapter. Uncertainties from well-log and petrophysical interpretations should be analyzed, quantified, and explicitly communicated to the integration team to be accounted for in reservoir characterization and modeling.