A rising tide: intrinsic alignments since the turn of the millennium

Abstract The alignments of galaxies across the large-scale structure of the Universe are known to be a source of contamination for gravitational lensing, but they can also probe cosmology and the physics of galaxy evolution in many ways. In this review, I cover developments in our understanding of intrinsic alignments over the past 25 years on: (1) different approaches to model intrinsic alignments across a range of scales, (2) existing observational constraints, (3) predictions from cosmological numerical N -body and hydrodynamical simulations, (4) mitigation strategies to account for their contamination to lensing observables and (5) cosmological and astrophysical applications. While the review focuses mostly on two-point statistics of intrinsic alignments, I also give a summary of other statistics beyond two-point. Finally, I point out some of the open problems hindering the understanding or application of intrinsic alignments and how they might be overcome in the future.