The absorptivity or reflectivity of single crystals of Nb has been measured from 0.1 to 36.4 eV. The data were Kramers-Kronig analyzed and the optical constants obtained. The absence of low-energy interband transitions has been noted, while interband effects have been observed beginning at about 1.4 eV and extending to about 19 eV. Comparisons with existing experimental data are made. Identification of some interband effects has been tentatively made from the band structure calculated by Mattheiss. The loss function has been obtained and strong resonances have been noted at 9.9 and 20.8 eV. From our spectra of the complex dielectric constant, we can identify both of these as being plasmon excitations. The onset of core transitions at about 28 eV has been observed.