The transition states for several insertion reactions have been determined at the 3-21G level of theory. While the insertions of CH2 and SiH2 into the CH and SiH bonds of methane and silane, respectively, all have nonzero SCF barriers, only the insertion of silylene into methane retains a nonzero barrier when third-order Moller-Plesset perturbation theory corrections are included with the 6-31G* basis set. The intrinsic reaction coordinates for the carbene and silylene insertions into methane are used to provide a pictorial view of these reactions and to relate the calculations to earlier studies. The insertion of silylene into H2 is analyzed at higher levels of theory in order to consider the relative importance of improved basis sets, multiconfigurational wave functions, and large-scale configuration interaction on the calculated barrier.