Abstract：With insufficient masses to sustain core hydrogen fusion, substellar objects continue to cool and fade after birth.  Those heavier than 13 jupiter masses, called brown dwarfs, manage to ignite deuterium or lithium, thereby maintaining hydrostatic equilibrium for a short period of time.   Those less massive than this do not undertake any nuclear reaction whatsoever in their lives and evolve like planets. So far a few thousand brown dwarfs and planetary-mass objects are known, almost all in the Galactic field in the solar neighborhood, i.e., they are already aged.   Searching for the youngest substellar objects by spectroscopy is hampered by their faintness and often confusion with foreground or background contaminations. We describe our efforts to identify substellar candidates in nearby star-forming regions of 1 to 2 Myr old, when brown dwarfs are being formed or in their infancy. Our sample of substellar populations in star clusters, with known ages and distances, provide stringent constraints on theoretical modeling of ultracool atmospheres, and of chromospheric activity. We also present how these least-massive members as the most vulnerable members in stellar dynamics to get ejected, leading to eventual disintegration of star clusters.