The optical properties of CrSi2, both in polycrystalline and single-crystal form, were investigated between 0.01 and 5 eV. The dielectric functions were determined by different methods: Kramers-Kronig transformations of the near-normal reflectivity over the whole spectral range; direct measurement by spectroscopic ellipsometry from 1.4 to 5 eV; numerical inversion of the reflectance from two films with different thickness. The main difference between thin-film and single-crystal data is the presence, in the latter, of a strong free-carrier response, preventing the determination of the intrinsic absorption edge (interband optical gap). Moreover, the optical properties of CrSi2 were calculated within the local-density approximation using the semirelativistic linear-muffin-tin-orbital method. The band structure, the l-projected densities of states, the complex dielectric function, and the optical reflectivity were obtained in the energy range from 0 to 5 eV. The theoretical calculations are compared with the experimental data.