Models of planetary growth are based upon data from our own Solar System, as well as observations of extrasolar planets and the circumstellar environments of young stars. Collapse of molecular cloud cores leads to central condensations (protostars) surrounded by higher specific angular momentum circumstellar disks. Planets form within such disks, and play a major role in disk evolution. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies—until the planetary orbits become separated enough that the configuration is stable for the age of the system. Giant planets begin their growth as do terrestrial planets, but they become massive enough to accumulate substantial amounts of gas before the protoplanetary disk dissipates. A potential hazard to planetary systems is radial decay of planetary orbits, resulting from interactions between the planets and the natal disk. Massive planets can sweep up disk material in their vicinity, eject planetesimals and small planets into interstellar space or into their star, and confine disks in radius and azimuth. Small planetary bodies (asteroids and comets) can sequester solid grains for long periods of time and subsequently release them.