  In planetary science, accretion is the process in which solids agglomerate to
form larger and larger objects and eventually planets are produced. The initial
conditions are a disc of gas and microscopic solid particles, with a total mass
of about 1% of the gas mass. Accretion has to be effective and fast. Effective,
because the original total mass in solids in the solar protoplanetary disk was
probably of the order of ~ 300 Earth masses, and the mass incorporated into the
planets is ~100 Earth masses. Fast, because the cores of the giant planets had
to grow to tens of Earth masses in order to capture massive doses of hydrogen
and helium from the disc before the dispersal of the latter, i.e. in a few
millions of years. There is probably not one accretion process but several,
depending on the scale at which accretion operates. A first process is the
sticking of microscopic dust into larger grains and pebbles. A second process
is the formation of an intermediate class of objects called planetesimals. A
third accretion process has to lead from planetesimals to planets. Actually,
several processes can be involved in this step, from collisional coagulation
among planetesimals to the accretion of small particles under the effect of gas
drag, to giant impacts between protoplanets. This chapter will detail all these
processes, adopting a historical perspective: i.e. from the classic processes
investigated in the past decades to those unveiled in the last years.
