Collective behaviours in chiral active systems. (A) The circle swimming algae E. voratum generates a period-averaged outgoing radial flow leading to a dispersion of the cells in a disordered hyperuniform state. Streak image averaged over 10 s. Reprinted with permission from [30]. Copyright©2021 National Academy of Sciences. (B) Anisotropic pear-shaped Quincke rollers powered by a static electric field favour rotations around the symmetry axis due to viscous drag leading to curved trajectories. Hydrodynamic alignment interactions then induce emergent patterns like vortices (image) or rotating flocks. Reprinted with permission from [50]. Copyright©2020 The Author(s). (C) Hydrodynamic interactions in an ensemble of isotropic rotors leads to a cascade of transverse dynamics and the formation of multi-scale clock-wise and counter-clock-wise vortices. Reprinted with permission from [34]. Copyright©2023 The Author(s). (D) Viscous edge pumping effect in a cohesive magnetic spinner fluid gives rise to unidirectional surface waves. Spectral decomposition of the surface fluctuations allowed the first experimental measurement of odd viscosity in a soft matter system. Reprinted with permission from [35]. Copyright©2019 Springer Nature. (E) Magnetic colloidal spinners with significant magnetic attraction form rotating and “kneading” polycrystalline structures resulting from the combination of magnetic and hydrodynamic interactions. Reprinted with permission from [43]. Copyright©2022 Springer Nature. (F) Spontaneous assembly of swimming starfish embryos (σ≈200 m) into a chiral active crystal featuring sustained overdamped odd elastic waves. Reprinted with permission from [32]. Copyright©2022 Springer Nature.