The unique properties of liquid water arise from its dynamic hydrogen-bond network with lifetimes of about one picosecond. Since an oscillating electric field with a picosecond period has a spectrum spanning the terahertz (THz) range, THz spectroscopy offers a powerful means to probe such transient molecular network, including collective modes such as “small and short-lived phonons”. The talk is structured in two parts. First, we explore how intense THz time-domain spectroscopy can pinpoint the interactions between water and various solutes. In the second part, we examine the initiation and consequences of photo-ionisation processes in liquid environments, focusing on charge-carrier interactions.