In 1990 Tuerk and Gold introduced the first RNA aptamer for bacteriophage T4 DNA polymerase, obtained by a new combinatorial technique which they designated as SELEX (systematic evolution of ligands by exponential enrichment). In parallel, ELLINGTON and Szostak (1990) showed that it is also possible to select RNA aptamers which are able to specifically complex organic molecules of low molecular weight, thus serving as receptor molecules based on nucleic acids rather than proteins. Since then, considerable progress has been achieved in the field of in vitro selection of combinatorial nucleic acid libraries, which demonstrates its impressive potential as a tool in molecular biology, diagnostics, molecular medicine, drug discovery, and bio-organic chemistry. Today, the SELEX process has been applied to more than a hundred different target molecules, and aptamers are known for almost every kind of targets such as organic dyes, amino acids, biological cofactors, antibiotics, peptides and proteins or even whole viruses (Bell et al. 1998; Gal et al. 1998; Ellington and Osborne 1997; Kraus et al. 1998; Yang et al. 1998; Eaton 1997; Pan et al. 1995), showing that aptamers can be obtained for almost any desired target whether complex or small.