Oligonucleotides (ONs) can interfere with biomolecules representing the entire extended central dogma. Antisense gapmer, steric block, spliceswitching ONs, and short interfering RNA drugs have been successfully developed. Moreover, antagomirs (antimicroRNAs), microRNA mimics, aptamers, DNAdecoys, DNAzymes, synthetic guide strands for CRISPR/Cas, and innate immunity-stimulating ONs are all in clinical trials. DNAtargeting, triplex-forming ONs and strand-invading ONs have made their mark on drug development research, but not yet as medicines. Both design and synthetic nucleic acid chemistry are crucial for achieving biologically active ONs. The dominating modifications are phosphorothioate linkages, base methylation, and numerous 2'-substitutions in the furanose ring, such as 2'-fluoro, O-methyl, or methoxyethyl. Locked nucleic acid and constrained ethyl, a related variant, are bridged forms where the 2'-oxygen connects to the 4'-carbon in the sugar. Phosphorodiamidate morpholino oligomers, carrying a modified heterocyclic backbone ring, have also been commercialized. Delivery remains a major obstacle, but systemic administration and intrathecal infusion are used for treatment of the liver and brain, respectively. Expected final online publication date for the Annual Review of Pharmacology and Toxicology Volume 59 is January 6, 2019. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.