Brown dwarfs are substellar objects unable to stably fuse hydrogen in their interior. Since the discovery of the first brown dwarf in an open cluster, namely, Teide 1 (Rebolo et al., Nature 377:129–131, 1995), many searches for substellar objects have been carried out in young stellar clusters and associations, such as Pleiades, Orion, Upper Scorpius, Taurus, Chamaeleon, α Persei, Hyades, or Praesepe. The lithium test has proven to be a very useful tool to distinguish between brown dwarfs and stars and to determine the ages of young open clusters. Young substellar objects show spectral features that are sensitive to surface gravity, which is expected to be lower than in older field counterparts of similar effective temperature. The studies of the substellar mass function indicate that brown dwarfs are very numerous, about one-third of the total number of stars, but their contribution in mass is lower than 10%. The formation of substellar objects extends below the deuterium-burning mass limit, which is the realm of the so-called free-floating or isolated planetary-mass objects that overlap with the masses of exoplanets.