The notion of quality of a finite element solution -- The constitutive relation error method for linear problems -- Other methods for linear problems -- Principles of the comparison of the various estimators in the linear case -- Mesh adaptation for linear problems -- The constitutive relation error method for nonlinear evolution problems -- The constitutive relation error method in dynamics -- Techniques for constructing admissible fields -- Estimation of local errors.Mastering Calculations in Linear and Nonlinear Mechanics is concerned with the management of calculations in linear and nonlinear mechanics. In the last twenty years or so this constant preoccupation in industry as well as in research has gone through a revolution with the advent of truly quantitative tools. Particular attention is given to error estimators and indicators for structural analysis, as this domain is the most mature today. The accent is on the concept of error in constitutive relation. An important part of this work is also devoted to the utilization of the error estimators involved in a calculation, beginning with the parameters related to the mesh. Many of the topics are taken from the most recent research by the authors: local error estimators, extension of the concept of error in constitutive relation to nonlinear evolution problems and dynamic problems, adaptive improvement of calculations in nonlinear mechanics. This work is intended for all those interested in mechanics: students, researchers and engineers concerned with the construction of models as well as their simulation for industrial purposes.