  Our understanding of planet formation has been rapidly evolving in recent
years. The classical planet formation theory, developed when the only known
planetary system was our own Solar System, has been revised to account for the
observed diversity of the exoplanetary systems. At the same time, the
increasing observational capabilities of the young stars and their surrounding
disks bring new constraints on the planet formation process. In this chapter,
we summarize the new information derived from the exoplanets population and the
circumstellar disks observations. We present the new developments in planet
formation theory, from dust evolution to the growth of planetary cores by
accretion of planetesimals, pebbles, and gas. We review the state-of-the-art
models for the formation of diverse planetary systems, including the population
synthesis approach which is necessary to compare theoretical model outcomes to
the exoplanet population. We emphasize that the planet formation process may
not be spatially uniform in the disk and there are preferential locations for
the formation of planetesimals and planets. Outside of these locations, a
significant fraction of solids is not growing past the pebble-sizes. The
reservoir of pebbles plays an important role in the growth of planetary cores
in the pebble accretion process. The timescale of the emergence of massive
planetary cores is an important aspect of the present models and it is likely
that the cores within one disk form at different times. In addition, there is
growing evidence that the first planetary cores start forming early, during the
circumstellar disk buildup process.
