  Generic interacting many-body quantum systems are believed to behave as
classical fluids on long time and length scales. Due to rapid progress in
growing exceptionally pure crystals, we are now able to experimentally observe
this collective motion of electrons in solid-state systems, including graphene.
We present a review of recent progress in understanding the hydrodynamic limit
of electronic motion in graphene, written for physicists from diverse
communities. We begin by discussing the "phase diagram" of graphene, and the
inevitable presence of impurities and phonons in experimental systems. We
derive hydrodynamics, both from a phenomenological perspective and using
kinetic theory. We then describe how hydrodynamic electron flow is visible in
electronic transport measurements. Although we focus on graphene in this
review, the broader framework naturally generalizes to other materials. We
assume only basic knowledge of condensed matter physics, and no prior knowledge
of hydrodynamics.
