  The ESA space mission Euclid was launched on July 1st, 2023 and is undergoing
its science verification phase. In this invited review we show that Euclid
means a before and an after for our understanding of ultra-cool dwarfs and
substellar-mass objects and their connections with stars, exoplanets and the
Milky Way. Euclid enables the study with unprecedented statistical significance
a very large ensemble of ultracool dwarfs, the identification of new types of
substellar objects, and the determination of the substellar binary fraction and
the Initial Mass Function (IMF) in diverse galactic environments from the
nearest stellar nurseries to the ancient relics of Galactic formation.
