This review discusses the observed properties of interstellar dust grains, including extinction of starlight, optical luminescence, infrared emission, microwave emission, scattering by dust, polarization of starlight and infrared emission, and the relationship between presolar grains in meteorites and interstellar grain populations. The review considers candidate grain materials and abundance constraints, and compares a dust model consisting of amorphous silicate grains, graphite grains, and polycyclic aromatic hydrocarbons with observed emission and scattering. It also discusses issues concerning the evolution of interstellar dust. The extinction of starlight by interstellar dust is studied through the wavelength-dependent attenuation, known as "reddening," which is influenced by grain size distribution and spectral features. The "pair method" is used to measure extinction curves for various sightlines, revealing variations in the extinction curve slope, represented by the quantity R_V. The extinction curve is often approximated by a seven-parameter function of wavelength, with parameters determining the strength and shape of the 2175Å "bump" and the slope and curvature of the continuous extinction. Dust in other galaxies is also studied, with the Large and Small Magellanic Clouds providing valuable data on extinction laws. The extinction law in the Milky Way is found to be similar to that in other galaxies, with variations depending on the galactic environment. The 2175Å feature is attributed to graphitic carbon or large polycyclic aromatic hydrocarbon (PAH) molecules, with PAHs being a likely candidate due to their similar electronic properties to graphite. Silicate features in the infrared are attributed to interstellar silicate material, with absorption peaks at 9.7 and 18 µm. The strength of these features relative to A_V is measured, and the interstellar silicates are believed to be largely amorphous rather than crystalline. The 3.4 µm feature is attributed to aliphatic C-H stretching in refractory grain material, while ice features in dense molecular clouds are due to H2O and other species. PAH features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm are identified as vibrational modes of PAH molecules. X-ray absorption edges are studied, showing significant structure near various atomic edges. Extended red emission (ERE) is attributed to photoluminescence, with the efficiency of interstellar dust estimated at 10%. Candidate materials for ERE include HAC, PAHs, and silicon nanoparticles. Presolar grains in meteorites are identified by their isotopically anomalous composition, with types including nanodiamonds, SiC grains, graphitic grains, Al2O3 corundum grains, and Si3N4 grains. These grains are believed to have formed in various astrophysical environments, such as supernovae and asymptotic giant branch stars. TheThis review discusses the observed properties of interstellar dust grains, including extinction of starlight, optical luminescence, infrared emission, microwave emission, scattering by dust, polarization of starlight and infrared emission, and the relationship between presolar grains in meteorites and interstellar grain populations. The review considers candidate grain materials and abundance constraints, and compares a dust model consisting of amorphous silicate grains, graphite grains, and polycyclic aromatic hydrocarbons with observed emission and scattering. It also discusses issues concerning the evolution of interstellar dust. The extinction of starlight by interstellar dust is studied through the wavelength-dependent attenuation, known as "reddening," which is influenced by grain size distribution and spectral features. The "pair method" is used to measure extinction curves for various sightlines, revealing variations in the extinction curve slope, represented by the quantity R_V. The extinction curve is often approximated by a seven-parameter function of wavelength, with parameters determining the strength and shape of the 2175Å "bump" and the slope and curvature of the continuous extinction. Dust in other galaxies is also studied, with the Large and Small Magellanic Clouds providing valuable data on extinction laws. The extinction law in the Milky Way is found to be similar to that in other galaxies, with variations depending on the galactic environment. The 2175Å feature is attributed to graphitic carbon or large polycyclic aromatic hydrocarbon (PAH) molecules, with PAHs being a likely candidate due to their similar electronic properties to graphite. Silicate features in the infrared are attributed to interstellar silicate material, with absorption peaks at 9.7 and 18 µm. The strength of these features relative to A_V is measured, and the interstellar silicates are believed to be largely amorphous rather than crystalline. The 3.4 µm feature is attributed to aliphatic C-H stretching in refractory grain material, while ice features in dense molecular clouds are due to H2O and other species. PAH features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm are identified as vibrational modes of PAH molecules. X-ray absorption edges are studied, showing significant structure near various atomic edges. Extended red emission (ERE) is attributed to photoluminescence, with the efficiency of interstellar dust estimated at 10%. Candidate materials for ERE include HAC, PAHs, and silicon nanoparticles. Presolar grains in meteorites are identified by their isotopically anomalous composition, with types including nanodiamonds, SiC grains, graphitic grains, Al2O3 corundum grains, and Si3N4 grains. These grains are believed to have formed in various astrophysical environments, such as supernovae and asymptotic giant branch stars. The