  At the most fundamental level, the interaction between light and matter is
manifested by the emission and absorption of single photons by single quantum
emitters. Controlling light--matter interaction is the basis for diverse
applications ranging from light technology to quantum--information processing.
Many of these applications are nowadays based on photonic nanostructures
strongly benefitting from their scalability and integrability. The confinement
of light in such nanostructures imposes an inherent link between the local
polarization and propagation direction of light. This leads to {\em chiral
light--matter interaction}, i.e., the emission and absorption of photons depend
on the propagation direction and local polarization of light as well as the
polarization of the emitter transition. The burgeoning research field of {\em
chiral quantum optics} offers fundamentally new functionalities and
applications both for single emitters and ensembles thereof. For instance, a
chiral light--matter interface enables the realization of integrated
non--reciprocal single--photon devices and deterministic spin--photon
interfaces. Moreover, engineering directional photonic reservoirs opens new
avenues for constructing complex quantum circuits and networks, which may be
applied to simulate a new class of quantum many--body systems.
