We study the stability and migration mechanism of self-interstitials in Si through first-principles self-consistent pseudopotential calculations. The neutral Si interstitial is lowest in energy at a -split site, with energy barriers of 0.15-0.18 eV for migrating into hexagonal and tetrahedral interstitial sites, while the migration barrier from a hexagonal site to a tetrahedral site is lower, 0.12 eV. These migration barriers are further reduced through successive changes in the charge state at different sites, which allow for the athermal diffusion of interstitials at very low temperatures. The -split geometry is also the most stable structure for negatively charged states, while positively charged seif-interstitials have the lowest energy at tetrahedral sites. Apart from the migration barrier, the formation energy of the -split interstitial is estimated to be about 4.19 eV; thus, the resulting activation enthalpy of about 4.25 eV is in good agreement with high-temperature experimental data.