We have investigated the atomic and electronic structures of hydrogen saturated silicon nanowires doped with impurity atoms (such as Al, Ga, C, Si, Ge, N, P, As, Te, Pt) using a first-principles plane wave method. We considered adsorption and substitution of impurity atoms at the surface and also their substitution at the core of the nanowire. In the case of adsorption to the surface, we determined the most energetic adsorption geometry among various possible adsorption sites. All impurities studied lead to nonmagnetic ground state with a significant binding energy. Impurity bands formed at high impurity concentration are metallic for group IIIA and VA elements but are semiconductor and modify the band gap for group IVA and VIA elements. While low substitutional impurity concentration leads to usual n - and p -type behaviors reminiscent of bulk Si, this behavior is absent if the impurity atom is adsorbed on the surface. It is shown that the electronic properties of silicon nanowires can be modified by doping for optoelectronic applications.