This chapter reports the methodologies to model and estimate the extent of two undesired but unavoidable phenomena related to membranes: the concentration polarization (i.e., a concentration gradient along the permeation direction) and inhibition, which is here studied only in the presence of CO, although other species such as CO2 and H2O can also be generally considered. The first part concerns the definition of an appropriate Concentration Polarization Coefficient (CPC) and Inhibition Coefficient (IC), which can be combined to define an overall Permeation Reduction Coefficient (PRC), providing an immediate measure of the permeation loss with respect to the best conditions (pure hydrogen). Along with such coefficients, the possibility of using another one called the Concentration Gradient Coefficient (CGC) is also discussed as a quantification of the concentration drop owing to permeation, different from CPC, which makes use of the characteristic Sieverts’ driving force. The second part focuses on approaches to estimate the concentration polarization in membrane reactors and contactors, describing the convenience of using the so-called Effective Average Concentration Polarization Coefficient (EAC), which measures the polarization level more correctly than the conventional average CPC. Such a coefficient provides direct information to scholars, scientists, and end-users to estimate the membrane effectiveness, in an analogous way to the effectiveness factor used to determine the efficiency of a catalyst.