Dramatic advances in understanding of the roles RNA plays in normal health and disease have greatly expanded over the past 10 years and have made it clear that scientists are only beginning to comprehend the biology of RNAs. It is likely that RNA will become an increasingly important target for therapeutic intervention; therefore, it is important to develop strategies for therapeutically modulating RNA function. Antisense oligonucleotides are perhaps the most direct therapeutic strategy to approach RNA. Antisense oligonucleotides are designed to bind to the target RNA by well-characterized Watson-Crick base pairing, and once bound to the target RNA, modulate its function through a variety of postbinding events. This review focuses on the molecular mechanisms by which antisense oligonucleotides can be designed to modulate RNA function in mammalian cells and how synthetic oligonucleotides behave in the body.