Many soft materials and biomolecules such as lipids, surfactants, polymers, and proteins self-assemble in water and form various aggregate structures. It is not fully understood whether the surrounding water plays an active role in the formation of these aggregate structures or is merely a background solvent. On the other hand, an attempt has recently been made to measure the dynamics of water molecules around soft materials using a technique called terahertz spectroscopyTerahertz spectroscopy and to accurately determine the hydrationHydration state from the dynamical changes. This technique allows the observation of water that is weakly affected by the solute whose dynamics is slightly slowed down. As a result, it has been found that “weakly affected waterWeakly affected water“spread over long distances on molecular surfaces is deeply involved in the formation of aggregate structures and phase behavior of soft materials and biomolecules.