Long hypothesized to be the origin of planetary systems, disks around newly formed stars have been studied in detail in the last twenty years. Most, if not all stars form surrounded by a disk, with typical masses of 0.001–0.3 M๏ and up to several hundred AU. Molecular emission lines show the gas to be in Keplerian rotation, with most species (but not H2) frozen out onto dust grains in the cold and dense disk interior. The fraction of stars with disks decreases from > 80% at < 1 Myr to < 10% at 10 Myr. The disk “half-life" is 2–3 Myr, with the inner and outer disks disappearing nearly simultaneously. There is a small but distinct populations of disks with cleared-out inner regions, so-called cold or transitional disks, explained by photoevaporation, planet formation, or binarity. Inside the disks, planet formation begins, with clear observational evidence for the growth of dust grains to sizes of a few cm at least.