  Raman spectroscopy is an integral part of graphene research. It is used to
determine the number and orientation of layers, the quality and types of edge,
and the effects of perturbations, such as electric and magnetic fields, strain,
doping, disorder and functional groups. This, in turn, provides insight into
all $sp^2$-bonded carbon allotropes, because graphene is their fundamental
building block. Here we review the state of the art, future directions and open
questions in Raman spectroscopy of graphene. We describe essential physical
processes whose importance has only recently been recognized, such as the
various types of resonance at play, and the role of quantum interference. We
update all basic concepts and notations, and propose a terminology that is able
to describe any result in literature. We finally highlight the potential of
Raman spectroscopy for layered materials other than graphene.
