Nucleic acids that can bind with high affinity and specificity to target molecules are called "apta mers". Aptamers recognise a large variety of different molecule classes. The main focus of this chapter is small molecules as targets. Aptamers are applied complementarily to antibody technologies and can substitute antibodies or small molecules wherever their different properties, such as biochemical nature or highly discriminating capacities, are advantageous. Examples of promising applications of these versatile molecules are discussed in the field of therapeutics and biotechnology with a special view to small-molecule detection.