This book describes the fundamental physical, chemical, molecular and biological aspects of proteins binding to one another in cellular biology. PPI is a common phenomenon in cell and molecular biology events. The networks of PPI describe cellular function in both unicellular and multi-cellular organisms. Homo complexes (dimers of identical monomers) or hetero complexes (dimers of non-identical monomers) are involved in PPI. These complexes are common in catalysis, regulation and structural assembly. However, the molecular principles of protein interactions are difficult to understand due to the geometrical and chemical characteristics of proteins. The challenge today is to identify interaction partners and sites from sequence and/or structures. This book describes the principles (physics and chemistry) of PPI and protein interfaces to develop models for PPI predictions.