The Cast3M software (https://www-cast3m.cea.fr) was initially developed in the Service of mechanical and thermal studies (SEMT) at the French Atomic Energy Commission (CEA). The development of Cast3M is done within the framework of a research activity in the field of mechanics, the goal of which is to build a high level computational tool, useful in designing, dimensioning and analysing 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 devoted to the computations themselves, but also incorporating preprocessing functions (meshing, importing) and postprocessing tools (visualization, export). All the program's capabilities are accessible through a simple, extensible, domain-specific language (DSL), Gibiane, that the user can use to specify and solve his problems. Cast3M can address a large range of applications: mechanical linear elastic problems in statics and dynamics (vibration, eigenvalue computation), thermal and heat transfer problems, nonlinear problems (plasticity, large rotation and deformation, contact, friction), step by step implicit dynamic problems, structural failure, fluid-structure interactions, etc... The Cast3M software is also open-source, multi-platform, freely downloadable for academic and research use. Training sessions at various levels and a user seminar take place every year. As year 2023 marks the 40th anniversary of the Cast3M platform, this talk will provide insights into its history, some lessons learned, present some current non-trivial applications and challenges for the future.