This paper discusses the ability of Li to confirm the substellar nature of a brown dwarf candidate. Theoretical computations using different interior models agree that brown dwarfs with masses below $sim$ 0.065 M$_{odot}$ preserve a significant fraction of their initial Li content while for higher masses total Li depletion occurs in very short timescales. Refined spectral synthesis using brown dwarf model atmospheres shows that the Li I resonance line at 670.8 nm should produce a conspicuous feature at effective temperatures higher than 2000 K, suitable for spectroscopic detection in present brown dwarf candidates. We summarize the results of our search for Li in many of the best candidates known so far.