  Bound states in the continuum (BICs) have attracted attention in photonics
owing to their interesting properties. For example, BICs can effectively
confine light in a counter-intuitive way and the far-field radiation of
photonic structures that exhibit BICs manifests fascinating topological
characteristics. Early research into photonic BICs was primarily focused on
designing artificial structures to produce BICs. However, since the mid-2010s,
exploring the potential applications of BICs has been a growing trend in
research. In this Review, we detail the unique properties of BICs, including
the ability to achieve enhanced light confinement, sharp Fano resonances, and
topological characteristics. We also explore phenomena derived from BICs
including the generation of circularly polarized states and unidirectional
guided resonances and the impact of BICs on various applications such as
lasing, nonlinear frequency conversion, waveguiding, sensing and wavefront
control. We also discuss the insights provided by BICs in several emerging
research frontiers, such as parity-time symmetric systems, higher-order
topology, exciton-photon coupling, and moir\'e superlattices.
