Aptamers have a huge potential as therapeutic agents. They can be addressed to intracellular targets, to membrane components or to circulating molecules. They can be used as drug-delivery agents to cells of interest. They can be employed as antagonists (preventing a specific receptor-ligand interaction), or as agonists (binding to a receptor and activating a signal). Aptamers can be easily modified for improving their properties (making them more resistant to nucleases or increasing their in vivo half life). They induce a minimal immune response in vivo. Several clinical trials are on-going.