Ab initio MO calculations for the concerted 1,3-silyl migration in allylsilane at Hartree-Fock and multiconfigurational levels revealed the existence of two transition structures (TS), which lead to retention and inversion of stereochemistry at the silicon. The two structures are pentacoordinate square pyramidal and trigonal bipyramidal around the silicon atom, respectively. The energy barriers estimated at various levels of calculation, including those with electron correlation effects, indicate that the retention transition structure is ca. 9 kcal/mol more stable than the inversion transition structure, in contrast to the experimental results of Kwart et al. Introduction of a pi system, such as a vinyl group, at the silicon stabilizes the inversion TS more than the retention TS. Similar transition structures and energetics were obtained for 1,3-migration in allylgermane and allylstannane using effective core potentials for Ge and Sn.