  A Monte Carlo framework, MaGe, has been developed based on the Geant4
simulation toolkit. Its purpose is to simulate physics processes in low-energy
and low-background radiation detectors, specifically for the Majorana and Gerda
$^{76}$Ge neutrinoless double-beta decay experiments. This jointly-developed
tool is also used to verify the simulation of physics processes relevant to
other low-background experiments in Geant4. The MaGe framework contains
simulations of prototype experiments and test stands, and is easily extended to
incorporate new geometries and configurations while still using the same
verified physics processes, tunings, and code framework. This reduces
duplication of efforts and improves the robustness of and confidence in the
simulation output.
