A circumstellar disk of gas and dust surrounding a pre-main sequence star from which planetary systems form. Protoplanetary disks are remnants of accretion disks which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10^-2 solar masses, lifetimes ~10^6 - 10^7 years, and accretion rates ~10^-7 - 10^-8 solar masses per year. According to the standard theory of planet formation, called core accretion, planets come into being by the growth of dust grains which stick together and produce ever larger bodies, known as planetesimals. The agglomeration of these planetesimals of 100 to 1000 km in size into rocky Earth-mass planets is the main outcome of this theory. Beyond the snow line in the disk, if the masses of these cores of rock and ice grow higher than 10 times that of Earth in less than a few million years, gas can rapidly accrete and give rise to giant gaseous planets similar to Jupiter. If core building goes on too slowly, the disk gas dissipates before the formation of giant planets can start. Finally the left-over planetesimals that could not agglomerate into rocky planets or core of giant planets remain as a debris disk around the central object that has become a main sequence star. An alternative to core accretion theory is formation of planets in a massive protoplanetary disk by gravitational instabilities. The validity of these two theories is presently debated.