The potential energy surfaces for the insertion reactions of germylene into XHn molecules have been characterized in detail using ab initio molecular orbital theory and density functional theory. The model system Ge(CH3)(2) + XHn (X = C, N, O, F, Si, P, S, and Cl; n = 1-4) has been chosen for the present study. All the interactions involve the initial formation of a donor-acceptor complex, followed by a high-energy transition state, and then an insertion product. The agreement between MP2 and B3LYP results indicates that the latter provides an adequate theoretical level for further investigations of molecular geometries, electronic structures, and kinetic features of the germylene reactions. The following conclusions emerge from this work: (i) the X-H insertion reactions of germylene occur in a concerted manner via a three-membered-ring transition state, and that the stereochemistry at the heteroatom X center is preserved; (ii) the stabilization energies of the germylene-XHn complexes increase in the order NH3 > H2O > PH3 > H2S similar to HF > HCl much greater than SiH4 similar to CH4; (iii) the order of reactivity for X-H bonds toward germylene insertion is Cl > F > S > O > P > N much greater than Si > C. In other words, the greater the atomic number of heteroatom (X) in a given row, the easier the insertion reaction of XHn hydrides and the larger the exothermicity. Moreover, the present study demonstrates that both electronic and steric effects play a major role in the course of insertion reactions of germylene into X-H bonds. This work also indicates that the chemical behavior of germylene should be more similar to that of silylene than to that of carbene species.