custEM is an open-source toolbox for customizable 3D finite-element modeling of controlled-source electromagnetic data. The toolbox is based on the finite- element library FEniCS, see https://fenicsproject.org/ . Different total and secondary electric or magnetic field as well as gauged-potential approaches are implemented. In addition, custEM contains a mesh generation submodule for the straightforward generation of tetrahedral meshes supporting a large number of marine, land-based, airborne or mixed model scenarios. Interpolation and visualization tools simplify the post-processing workflow.