The term damage can be generally defined as a change introduced into a system that affects its performance. Its identification and characterization is a valid help in deciding amongst continuing the operation or performing a repair or replacement of the system. A valid support to this decision is based on the use of well-known measurement techniques from Non-Destructive Testing and Evaluation (NDT&E). A well-established correlation between damage and features extracted from the measured data makes these techniques capable of providing information about the extent of the damage. However the prediction of the remaining useful life of a system by comparing full-filed measurements techniques and FEM analysis results is the challenge of an increasing number of research studies. The need of a guide for enumerating the extent of the damage has been the thrust to perform this work. A common methodology developed for both numerical and experimental studies will be presented. It consists of three main parts: proper selection of the parameter capable of describing the damage in a quantitative manner; several approaches to obtain results from measurement techniques and FEM analysis; and damage assessment making use of a quantitative comparison of FEM results only, full-field experimental results only, or comparison of FEM to experimental results. A different approach in damage assessment will be also presented making use of Zernike moment descriptors from which the severity of the damage is inferred. An example to illustrate the methodology will be shown.