With the advances in molecular biology and biochemistry new imaging and treatment modalities based on the biological properties of tissues have been developed. In oncology, the major progress has been achieved using peptide and antibody targeting vectors. Besides the identification of new target structures, progress in molecular biology also made new techniques for the development of new biomolecules. This relies on the identification of lead compounds and on the screening of various derivatives of these compounds one at a time. The principle of high-troughput methods for the identification of novel high affinity binders is to generate a vast library of possible variants of the molecule of interest and screen the population for the few variants that show the property of interest. The attracting feature of the concept arises from the huge number of candidate molecules that can be used for further evaluation. After the characterization of the structure-function relationships for the lead compounds found in this process further improvement by rational design of analogs can be performed.