Aptamers are folded oligo(deoxy)nucleotides that interact with designated target molecules with high affinity and selectivity. They have been generated binding to small molecules, toxins, peptides, proteins, viruses, bacteria, and even whole cells. Aptamers are less expensive than antibodies, non-to low toxic or immunogenic and provide specific binding properties. Aptamers show no batch-to-batch variations, attributed to their production by standardized chemical synthesis. They are about ten times smaller than antibodies, which improves their access to tissues and cells. These features and other characteristics, e.g. adaptive target recognition and compatibility with cellular transcription machineries make aptamers a valuable alternative to protein based detector and effector reagents. A variety of aptamer modifications are available in order to increase aptamer half-lives and to adapt aptamers towards any given application.