An operational definition of aptamers could be given as “relatively small oligonucleotides or peptides that bind their cognate targets based on their higher order structure rather than primary sequence”. One can argue that the higher order structures of these molecules are engrailed in the primary sequence of their residues. There are two approaches to designing or finding a cognate partner for a given ligand also referred to as combinatorial or rational approaches. Combinatorial process involves affinity selection of the binding molecule from among a pool of diverse molecules as in whole extracts of plants and animals or in intermediate soups of un directed and unprotected synthesis. Nucleic acids offer the added convenience of chemical synthesis of libraries of over 10 different sequences, and re-amplification of selected sequences in vitro. This has accelerated the discoveries on new aptamers at a pace difficult to cope with. Cognate molecules find their applications in all spheres of clinical activity including diagnosis, prophylaxis and therapy to mention a few. Many of these applications are inseparable and hence one may find them distributed in different sections. What is not being mentioned here is the quest for novel catalytic RNA.