Aptamers are nucleic acid molecules that bind specific ligands. Barriers to the application of aptamers as therapeutic and diagnostic reagents have been overcome in the past several years. In particular, aptamers that bind biomedically relevant targets have proven to be efficacious at modifying cellular metabolism. Such aptamers can be stabilized by chemical modifications and potentially used in vivo. Researchers have begun to devise aptamer-based diagnostic assays that may rival more conventional immunoassays.