It is becoming increasingly evident that liquids and solutions are far from homogeneous and are structured on lengths scales from supramolecular to mesoscopic. Such structure ranges from hydrogen-bonded clusters in water, through pre-nucleation clusters in saturated solutions and mesoscopic structures in room-temperature ionic liquids, to macroscopic phase separation associated with liquid-liquid phase transitions. This gives rise to dynamics over a huge range of timescales ranging from femtoseconds to kiloseconds presenting a challenge to experiment and theory. Many aspects of liquid structuring such as the proposed presence of a second critical point in the supercooled phase of liquid water or the macroscopic phase separation of molecular liquids due to a liquid-liquid phase transition have proven to be controversial. However, these liquid structuring issues play a defining role in determining chemical reactivity, transport properties, crystal nucleation, and other physicochemical properties important to engineering and biology