One of the major goals in molecular medicine is to understand the basis of human diseases at the molecular level and to translate this information into new strategies for diagnosis and therapy. Peptide aptamers represent a novel generation of molecules, which are selected for their intracellular binding to a given target protein. They are useful tools for basic science in blocking the intracellular function of a target protein with high specificity, thereby allowing the study of distinct physiological and pathological processes within living cells. In addition, peptide aptamers provide a basis for the development of novel diagnostic and therapeutic strategies, with implications for a broad variety of different disease entities, including metabolic disorders, infections, and cancer.