The mechanisms for the insertions of atomic (ID and 3P) carbon and silicon into the C-Hand Si-H bonds of methane and silane are investigated by ab initio SCF methods, many body perturbation theory, and a localized molecular orbital (LMO) analysis. The LMO analysis shows that the insertion of 1D atoms into CH4 and SiH4 may be classified into two types: cationic hydrogen transfer and anionic hydrogen transfer. For the triplet atoms the LMO analysis suggests two different insertion reaction mechanisms: the near abstraction and the pull-push mechanisms.