The textbook "Protein - Protein Interaction" offers a comprehensive introduction to the complex networks of protein-protein interactions (PPI), which are crucial for understanding biological systems. It highlights the development of interactomics, a relatively new field of research that has emerged from the increasing importance of molecular biology and biochemistry in basic research as well as in disease prognosis and prevention. The book discusses how genetic variations and selective pressure influence interactions between biological components and how these interactions determine the behavior and morphology of organisms. Through quantitative experiments, it aims to understand complex biological systems and translate their dynamic networks into predictive models. The characterization of protein interactions is emphasized as key to understanding many biological processes.