Aptamers are oligonucleotides identified within a library of randomly synthesized candidates of large complexity (up to 10e 15 different candidates).They are obtained via a combinatorial process called SELEX (Systematic Evolution of Ligands by EXponantial enrichment).Aptamers targeted to any kind of molecules (amino acids,vitamins,proteins,nucleic acids) are ligands of high atTnity and exquisite specificity due to their 3D structure.Selected RNA or DNA molecules are amenable to chemical modification for improved nuclease resistance and increased in vivo lifetime.They can be easily conjugated to various pending groups by conventional nucleic acid chemistry on solid support.We developed aptamers that target viral RNA structures.We demonstrated that such aptamers inhibit in vitro key steps of the HIV-1 and HCV cycle.Aptamers have also been raised against a marker of tumors that were converted into tools for imaging.Therefore aptamers show a promising potential for both diagnostic and therapeutic applications.