Locked nucleic acid (LNA) is the term for oligonucleotides that contain one or more nucleotide building blocks in which an extra methylene bridge fixes the ribose moiety either in the C3'-endo (beta-D-LNA) or C2'-endo (alpha-L-LNA) conformation. The beta-D-LNA modification results in significant increases in melting temperature of up to several degrees per LNA residue. The alpha-L-LNA stereoisomer, which also stabilizes duplexes, lends itself to use in triplex-forming oligonucleotides and transcription factor decoys, which have to maintain a B-type (C2'-endo) DNA conformation. LNA oligonucleotides are synthesized in different formats, such as all-LNA, LNA/DNA mixmers, or LNA/DNA gapmers. Essentially, all aspects of antisense technology have profited from LNA due to its unprecedented affinity, good or even improved mismatch discrimination, low toxicity, and increased metabolic stability. LNA is particularly attractive for in vivo applications that are inaccessible to RNA interference technology, such as suppression of aberrant splice sites or inhibition of oncogenic microRNAs. Furthermore, the extreme antisense-target duplex stability (formation of persistent steric blocks) conferred by beta-D-LNA also contributes to the capacity to invade stable secondary structures of RNA targets. The in vivo studies reported so far indeed point to LNA as a promising antisense player at the horizon of clinical applications.