The Aptamers Ligands: Powerful Tools from Molecular Evolution discusses these short, single strand oligonucleotides that can fold in a three-dimensional conformation and how they can provide a precise molecular recognition of a given target. These nucleic acid based ligands are identified from large naive combinatorial oligonucleotide libraries using a well-established technology called SELEX (systematic evolution of ligands by exponential enrichment). Aptamers can be extremely specific towards their target molecule with high selectivity and proper affinity dissociation constants. Moreover, they can be selected to recognize a high diversity of molecules, from very small compounds, to large macromolecules, such as proteins.