Publisher Summary The integration of instrumentation and a numerical computational process on a structure offers the opportunity to enhance its classical performances. In an active control purpose, the synthesis of performant control laws remains a crucial step, which not only involves automatic competencies but also mechanical ones. The efficiency of an active control strategy for flexible smart structures relies at least on the quality of model. The method is based on the concept of constitutive relation error (CRE) used for iterative parametric model updating purposes. The CRE approach is based on the division of the equations of the initial problem into two groups: (1) the reliable equations (mechanic equilibrium equation, electric equilibrium equation, and boundary conditions) and (2) the less reliable equations (constitutive relations). The chapter presents an updating method based on the concept of error in constitutive relation for coupled mechanic-piezoelectric structures used in active control systems.