Coherent structures are recurrently recovered in transitional and turbulent shear flows. Stability and resolvent analyses are often able to explain their development and recurrence. However, it is still unclear under which conditions these simplified theories can be applied to turbulent flows, where coherent structures develop on top of random fluctuations at different scales, or to complex, three-dimensional flows. The purpose of this colloquium is be to bring together researchers studying the development of instabilities leading to coherent structures in different transitional or turbulent flows, trying to find common features between the mechanisms triggering or sustaining turbulence in such different cases.