In The Aptamer Handbook, leading scientists from academia as well as biotech and pharma companies introduce the revolutionary concept of designing RNA and DNA oligonucleotides with novel functions by in vitro selection. These functions comprise high affinity binding (aptamers), catalytic activity (ribozymes and deoxyribozymes) or combinations of binding and catalytic properties (aptazymes). Basic concepts and technologies describing in detail how these functional oligonucleotides can be identified are presented. Numerous examples demonstrate the versatility of in vitro selected oligonucleotides. Special emphasis has been put on a section that shows the broad applicability of aptamers, e. g. in target validation, for analytics, or as new therapeutics. This first overview in the field is of prime interest for a broad audience of scientists both in academia and in industry who wish to expand their knowledge on the potential of new oligonucleotide functions and their applications.