Cast3M software [1] has been developed at the Service of Mechanical and Thermal studies (SEMT) of the French Atomic Energy Commission (CEA) for over 40 years. This development is part of a research activity in the field of mechanics, the aim of which is to build a high-level calculation tool, useful for the design, dimensioning and analysis of structures and components, in the nuclear field as well as in more traditional industrial fields. As such, Cast3M is a complete integrated system, not only dedicated to the calculations themselves (solvers), but also integrating pre-processing functions (meshing, import of external data/meshes, definition of numerous finite element formulations, vast possibilities of boundary conditions) and post-processing tools (field manipulation, visualization, data export). All the program's functionalities are accessible via a simple high-level language, Gibiane, which users can use to specify and solve their problems, giving them access to all the data in their problem, making it a “white box” code. Cast3M can handle a wide range of applications: elastic linear mechanical problems in statics or dynamics (vibrations, eigenvalue calculations), thermal and heat transfer problems, non-linear problems (plasticity, damage, creep, fracture, large rotations and strains, contact, friction), weak or strong coupled problems (thermomechanics, porous medium, …), dynamic problems using implicit time analysis or modal-based decomposition, structural failure, instabilities (buckling and post-buckling analysis, snap-back or snap-through), fluid-structure interactions, etc. Although Cast3M's core business is the thermomechanics of structures, it can also deal with other physics such as diffusion, electromagnetism, additive manufacturing, metallurgy or even topology optimization. Cast3M software is open source, multi-platform and freely downloadable for academic and research purposes. Training sessions for beginner and advanced users, are organized every year, as well as a user seminar. This presentation gives an overview of the code's main functionalities, briefly presents how it is used and illustrates current application cases. [1] Cast3M web site: https://www-cast3m.cea.fr