Recent advances in molecular biology have improved our understanding of basic pathophysiological mechanisms, and indicated possible strategies for their manipulation. The use of oligonucleotides occupies a prominent position among the emerging strategies, and has given rise to a new generation of pharmaceutical preparations now entering clinical usage. Antisense oligonucleotides are capable of blocking the expression of specific genes, thus permitting highly precise intervention in pathophysiology at the molecular level. Several clinical trials have recently produced highly promising results. The most likely applications of antisense agents are viral infections, malignancies, and chronic inflammatory disease. The article consists in a presentation of the antisense principle, discussion of its advantages and limitations, and a review of recent clinical developments and future prospects.