Aptamers are nucleic acid ligands which are isolated from combinatorial oligonucleotide libraries by in vitro selection. They exhibit highly complex and sophisticated molecular recognition properties and are capable of binding tightly and specifically to targets ranging from small molecules to complex multimeric structures. Besides their promising application as molecular sensors, many aptamers targeted against proteins are also able to interfere with the proteins' biological function. Recently developed techniques facilitate the intracellular application of aptamers and their use as in vivo modulators of cellular physiology. Using these approaches, one can quickly obtain highly specific research reagents that act on defined intracellular targets in the context of the living cell.