Nucleic acid aptamers, like monoclonal antibodies, bind to their target molecule with high affinity and specificity. Owing to their molecular recognition properties that are based on their three-dimensional structure, aptamers have been used as powerful tools in various fields, such as detection, separation or purification of target molecules. In addition, their intrinsic characteristics make aptamers excellent candidates for therapeutic applications. Due to their high specificity, stability, low toxicity and apparent non-immunogenicity, they provide viable alternatives to antibody- and small molecule-based therapeutics. Unlike antibodies, they can be generated against toxic, unstable or difficult to purify proteins. Furthermore, they can be chemically derivatised to extend their bioavailability and lifetimes in biological fluids, and they even offer the possibility for fast and efficient regulation. Similar to other therapeutic nucleic acids, such as antisense oligonucleotides, ribozymes or siRNA, problems with cellular uptake and susceptibility to nucleolytic attack have to be overcome for their successful application. With the first aptamer-based drug having entered the market and a few in Phase II trials, the road has been paved for more to follow. In this review, the requirements and properties of therapeutic aptamers are discussed and the recent progress in aptamer research towards drug development is highlighted.