This chapter is devoted to the use of Digital Image Correlation (DIC) in solid mechanics. Imaging mechanical tests, or performing them in situ (i.e., within an imaging device), has opened the way to a quantitative analysis of the specimen kinematics and, thereby, of its mechanical properties. This technique is emblematic of inverse problems. As such, many different variants exist that provide fair solutions. However, the concept of optimality, in the sense of minimization of uncertainty due to acquisition noise, is a powerful guideline to achieve the least uncertain measurement driven by a final goal. Identification of constitutive parameters is first discussed to highlight the basic concepts of optimal comparison metrics and measurement uncertainty properties. Following the same methodology, different degrees of prior knowledge of the studied specimen or structure lead to different variations of DIC (Integrated DIC, Finite-Element DIC, or regularized DIC). These approaches are illustrated with various examples.