  In this chapter, we review the processes involved in the formation of
planetesimals and comets. We will start with a description of the physics of
dust grain growth and how this is mediated by gas-dust interactions in
planet-forming disks. We will then delve into the various models of
planetesimal formation, describing how these planetesimals form as well as
their resulting structure. In doing so, we focus on and compare two paradigms
for planetesimal formation: the gravitational collapse of particle
over-densities (which can be produced by a variety of mechanisms) and the
growth of particles into planetesimals via collisional and gravitational
coagulation. Finally, we compare the predictions from these models with data
collected by the Rosetta and New Horizons missions and that obtained via
observations of distant Kuiper Belt Objects.
