  Models of planet formation are built on underlying physical processes. In
order to make sense of the origin of the planets we must first understand the
origin of their building blocks. This review comes in two parts. The first part
presents a detailed description of six key mechanisms of planet formation: 1)
The structure and evolution of protoplanetary disks 2) The formation of
planetesimals 3) Accretion of protoplanets 4) Orbital migration of growing
planets 5) Gas accretion and giant planet migration 6) Resonance trapping
during planet migration. While this is not a comprehensive list, it includes
processes for which our understanding has changed in recent years or for which
key uncertainties remain.
  The second part of this review shows how global models are built out of
planet formation processes. We present global models to explain different
populations of known planetary systems, including close-in small/low-mass
planets (i.e., super-Earths), giant exoplanets, and the Solar System's planets.
We discuss the different sources of water on rocky exoplanets, and use
cosmochemical measurements to constrain the origin of Earth's water. We point
out the successes and failings of different models and how they may be
falsified.
  Finally, we lay out a path for the future trajectory of planet formation
studies.
