  In this review, we give an interdisciplinary overview of Bose-Einstein
condensation phenomena in photonic systems. We cover a wide range of systems,
from lasers to photon condensates in dye-filled cavities, to excitons in
semiconductor heterostructures, to microcavity polaritons, as well as emerging
systems such as mode-locked lasers and classical light waves. Rather than
diving into the specific properties of each system, our main focus will be to
highlight those novel universal phenomena that stem from the
driven-dissipative, non-equilibrium nature of these systems and affect the
static, dynamic and coherence properties of the condensate. We conclude with
our view on the future perspectives of this field for both fundamental science
and technological applications.
