A decade has passed since the introduction of locked nucleic acid (LNA) [1][2][3]. This chapter provides a review and a current status on the features and applications of LNA. Attention is focused on the structural chemistry of LNA and on its applications for therapeutics and probes. With regards to the latter point, the chapter is based on former comprehensive reviews on LNA applications [4][5][6] and, for that reason, is focused on results reported subsequently, with no intention of being comprehensive.The field of nucleic acid chemistry has evolved considerably during the past few decades, leading from synthetic developments via molecular recognition into nucleic acid-based therapeutics, powerful diagnostic probes, and modern nanobiotechnology. From the structural perspective, the central issue has been the thorough understanding of the basis for molecular recognition and the selective formation of nucleic acid complexes, initially the classic Watson-Crick-type double helix. The central creative challenge for synthetic chemists has been to design simple chemical perturbations of the natural duplex in order to increase our knowledge within nucleic acid chemical biology, and to bring this knowledge into applied nucleic acids research [7-9].The major motivation behind the design of nucleic acid analogues has, over the past two decades, been the antisense strategy aimed at the silencing of genes by strong binding towards mRNA, with or without associated RNA-cleavage. Two central problems have motivated the chemical effort: (1) the physiological instability of natural oligonucleotides; and (2) the relatively moderate affinity of short natural oligonucleotides for their RNA-targets [10,11]. If a solution to these problems -as well as to the delicate question of cellular delivery -can be found, then oligonucleotides will present unlimited therapeutic perspectives. Among the more than 1000 chemical analogues tested for these properties, a large part seems to induce a high degree of resistance towards nucleolytic degradation, although this is dependent on the sequence composition, the number and positioning of the chemical modifications within the sequence, and on the degree of perturbation compared to the natural chemical structure. On the other hand, significantly increasing the RNA-affinity of an Modified Nucleosides: in Biochemistry, Biotechnology and Medicine. Edited by Piet Herdewijn