  This review describes the emerging field of waveguide quantum electrodynamics
(WQED) concerned with the interaction of photons propagating in a waveguide
with localized quantum emitters. The collective emitter-photon interactions can
lead to both enhanced and suppressed coupling compared to the case of
independent emitters. Here, we focus on guided photons and ordered arrays,
leading to super- and sub-radiant states, bound photon states and quantum
correlations with promising quantum information applications. We highlight
recent groundbreaking experiments performed with different quantum platforms,
including cold atoms, superconducting qubits, semiconductor quantum dots,
quantum solid-state defects, and we provide a comprehensive introduction to
theoretical techniques to study the interactions and dynamics of these emitters
and the photons in the waveguide.
