I discuss the successful application of the "lithium test" to the problem of distinguishing young brown dwarfs from true stars, given that they occupy overlapping regions of effective temperature and luminosity. In simple terms, stars will burn lithium in at most 100 Myr, while brown dwarfs may never reach the core temperature required to do so. The minimum lithium burning mass is not the same as the minimum stellar mass however, introducing some subtleties into the lithium test. I discuss the history of this subject, leading up to its successful application in the Pleiades. In addition to verifying substellar status, observations of lithium can be used to assess the age of stars in clusters, which is helpful in the proper usage of the lithium test itself. I also discuss how lithium observations of very cool objects can be useful in constraining the nature of brown dwarf candidates both in the field and in star forming regions.
Brown Dwarfs and Extrasolar Planets
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