Empirically, intrinsic defects in SnO2. are known to give rise to a net oxygen substoichiometry and n-type conductivity; however, the atomistic nature of the defects is unclear. Through first-principles density functional theory calculations, we present detailed analysis of both the formation energies and electronic properties of the most probable isolated defects and their clustered pairs. While stoichiometric Frenkel and Schottky defects are found to have a high energetic cost, oxygen vacancies, compensated through Sn reduction, are predicted to be the most abundant intrinsic defect under oxygen-poor conditions. These are likely to lead to conductivity through the mobility of electrons from Sn(II) to Sn(IV) sites. The formation of Sn interstitials is found to be higher in energy, under all charge states and chemical environments. Although oxygen interstitials have low formation energies under extreme oxygen-rich conditions, they relax to form peroxide ions (O2 2-) with no possible mechanism for p-type conductivity.