Masonry is a composite material, and its behavior shows that its weaknesses lie in the minimum resistance of its components and the characteristics of the interfaces between them. Ceramic brick masonry has technological characteristics that make it suitable for housing and building functions. The bricks, of reduced dimensions and joined with mortars of variable characteristics, have the advantage of adapting to almost all construction projects considering the influence of the environment on their service life. The investigation of the structural behavior of masonry has had very significant advances in the laboratory during the last mid-century, which has allowed numerical modeling of the behavior of the material and validation of failure modes under seismic actions. The behavior of heritage masonry with thick walls differs greatly from simple masonry using conventional techniques and materials. These differences in behavior have only been confirmed through numerical simulation contrasted with experimental research. This chapter presents the numerical modeling used for simple and confined masonry with reinforced concrete and for very thick heritage masonry, using the finite element method validated with full-scale laboratory experiences.