  Ultraluminous X-ray sources (ULXs) are extreme X-ray binaries shining above
10^39 erg/s, in most cases as a consequence of super-Eddington accretion onto
neutron stars and stellar-mass black holes accreting above their Eddington
limit. This was understood after the discovery of coherent pulsations,
cyclotron lines and powerful winds. The latter was possible thanks to the
high-resolution X-ray spectrometers aboard XMM-Newton. ULX winds carry a huge
amount of power owing to their relativistic speeds (0.1-0.3 c) and are able to
significantly affect the surrounding medium, likely producing the observed 100
pc ULX superbubbles, and limit the amount of matter that can reach the central
accretor. The study of ULX winds is therefore quintessential to understand 1)
how much and how fast can matter be accreted by compact objects and 2) how
strong is their feedback onto the surrounding medium. This is also relevant to
understand supermassive black holes growth. Here we provide an overview on this
phenomenology, highlight some recent, exciting results and show how future
missions such as XRISM, eXTP and ATHENA will improve our understanding.
