In this work we report the results of ab initio electronic structure calculations for pure SnO2 as well as for some defects, such as the oxygen vacancy and the interstitial tin impurity, and for the In and Sb as substitutional impurities in a configuration which corresponds to a concentration of 4.2%. We also show results for related optical properties which are derived directly from the band structure. Our findings are in good agreement with the available data, and the role of the studied systems is described in the present work.