This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical luminescence; infrared emission; microwave emission; optical, UV, and X-ray scattering by dust; and polarization of starlight and of infrared emission. The relationship between presolar grains in meteorites and the interstellar grain population is discussed. Candidate grain materials and abundance constraints are considered. A dust model consisting of amorphous silicate grains, graphite grains, and polycyclic aromatic hydrocarbons is compared with observed emission and scattering. Some issues concerning evolution of interstellar dust are discussed.