The reaction of ammonia (NH3) on the Ge(100)-2 x 1 surface is investigated using density functional theory (DFT). We find that NH3 adsorbs molecularly onto Ge(100)-2 x 1 via the formation of a dative bond. The calculations also show that, unlike Si(100)-2 x 1, the activation barrier for subsequent dissociation of NH3 adsorbed on Ge(100)-2 x 1 is higher than that of reversible desorption, which indicates that NH3 has a low reactive sticking probability on the Ge(100)-2 x 1 surface. We also predict that nitrogen insertion into the Ge-Ge dimer requires NH3 overexposure because the activation barrier for NH2 insertion into the Ge-Ge dimer is significantly above the entrance channel. The nitridation reaction pathway results in the N-H bridge-bonded state, which is found to be 17.4 kcal/mol more stable than the reactants. We find that the reactions of NH3 on the Ge(100)-2 x 1 surface generally involve higher activation barriers and less stable intermediates than the analogous reactions on the Si(100)-2 x 1 surface.