Our open access Journals in Mechanical Engineering & Mechanics cover topics such as Modelling and Simulation, Mechanical and Materials Engineering, Friction, Mechanics of Advanced Materials, and Visualization in Engineering, to name a few. The use of experimental data in association with simulation models has become an active research topic. Indeed, new experimental facilities (such as digital image/volume correlation (DIC/DVC)) now enable to collect a large and diversified amount of data, and these may be used to identify and validate complex models, or to enhance predictions made by simulations tools. Unfitted Finite Element Methods (UFEM e.g. CutFEM, XFEM, Finite Cell FEM, CGFEM) have been extensively investigated over the last two decades. The use of computational meshes that do not need to conform to the geometrical features of boundary value problems is highly interesting to practitioners in many fields of computational sciences and engineering. However, UFEM are yet to be adopted as a standard analysis tool. This is due to a number of important theoretical, implementational and numerical challenges, which we wish to address in this special issue.