  A plasma becomes quantum when the quantum nature of its particles
significantly affects its macroscopic properties. To answer the question of
when the collective quantum plasma effects are important, a proper description
of such effects is necessary. We consider here the most common methods of
description of quantum plasma, along with the related assumptions and
applicability limits. In particular, we analyze in detail the hydrodynamic
description of quantum plasma, as well as discuss some kinetic features of
analytic properties of linear dielectric response function in quantum plasma.
We point out the most important, in our view, fundamental problems occurring
already in the linear approximation and requiring further investigation.
(submitted to Physics-Uspekhi)
