  In these lectures I highlight some key features of massive neutrinos in the
context of cosmology. I first review the thermal history and the free-streaming
kinematics of the uniform cosmic background neutrinos. I then describe how
fluctuations in the phase space distributions of neutrinos and other particles
arise and evolve after neutrino decoupling according to the linear perturbation
theory of gravitational instability. The different clustering properties of
massive neutrinos (aka hot dark matter) and cold dark matter are contrasted.
The last part discusses the nonlinear stage of gravitational clustering and
highlights the effects of massive neutrinos on the formation of cosmological
structure.
