For specific electromagnetic problems, it is always convenient to solve subsets or special cases of Maxwell’s equations, resulting in different electric, magnetic, or electromagnetic formulations. Based on these formulations, COMSOL Multiphysics has several electromagnetics modules (for instance, the AC/DC Module, RF Module, and Wave Optics Module), as well as several different physics interfaces for each module. A formulation is usually based on a vector field, a potential, or a combination of them. The vector fields and potentials are the dependent variables required to solve. Depending on specific cases, different numerical methods and technologies can also be used, resulting in more variants of the physics interfaces. This section aims to provide an overview of the main formulations, methods, and technologies used by AC/DC interfaces.