A .fem file is a specialized data format used primarily in the field of Computer-Aided Engineering (CAE) and Finite Element Analysis (FEA). These files contain mathematical representations of physical structures, including geometry, mesh data consisting of nodes and elements, material properties, and boundary conditions such as loads, pressures, and constraints. The format is most commonly associated with Altair HyperMesh and the OptiStruct solver, though it is also frequently used by other simulation software like FEKO for electromagnetic analysis. Engineers use .fem files to simulate how a physical product or component will react to real-world forces, vibration, heat, fluid flow, and other physical effects. The file acts as an input deck for numerical solvers, which process the complex differential equations defined within the mesh to predict structural integrity, fatigue, or performance. Because these files often contain critical proprietary engineering data, they are essential in industries like aerospace, automotive, and civil engineering for virtual prototyping and validation before physical manufacturing begins. The format can be either ASCII text-based, allowing for manual editing of the solver cards, or binary for more efficient storage of large-scale simulation models.