The selection and concomitant affinity maturation of proteins to bind to user-defined target molecules have become a key technology in biochemical research, diagnostics, and therapy. One of the most potent selection technologies for such applications is ribosome display. It works entirely in vitro, and this has two important consequences. First, since no transformation of any cells is required, libraries with much greater diversity can be handled than with most other techniques. Second, since a library does not have to be cloned and transformed, it is very convenient to introduce random errors in the library by PCR-based methods and select improved binders. Thus, a true directed evolution, an iteration between randomization and selection over several generations, can be conveniently carried out, e.g., for affinity maturation. Ribosome display has been used successfully for the selection of antibody fragments and other binding proteins, such as Designed Ankyrin Repeat Proteins (DARPins).