Water plays versatile roles in biological systems by altering its characteristics depending on the milieu it is placed. Here, I demonstrate distinctive dynamic characteristics in the water molecules at a protein–protein and protein-DNA interfaces that turn out to be a key to understanding the binding free energy. Molecular dynamics simulations revealed the presence of extremely slow interfacial water dynamics even at 300 K. We rationalize their emergence by constructing a thermodynamic– dynamic relationship diagram of the hydration water, and argue that it is generally expected at the interface comprising the binding surfaces of electrostatic complementarity. We illustrate the implications of the slow interfacial water by quantifying its contribution to the binding affinity. In this talk, I will present the detailed concepts and applications of the thermodynamical and dynamical aspects of water in biology.