At water’s boiling point, two different phases are stable at the same time: liquid water is in equilibrium with steam. A phase transition is a dramatic change in a system’s physical properties. When you heat water by 2°C at 99°C, it boils. Water transforms from a liquid to a gas, decreasing its density 1600-fold, losing its liquid properties. A small change in temperature drives a huge change in density. In contrast, if you heat water by 2°C at 25°C, water remains a liquid and its density decreases by only 0.2%. The dramatic change in density at 99°C is a phase transition. Other phase transitions among different states of matter include freezing, the magnetization of metals, solubilization and precipitation in liquids, the formation of membranes and micelles, and sharp transitions in the alignments of liquid crystal molecules (used in watches and computer displays). While the term ‘phase transition’ usually applies to macroscopic systems, ‘cooperativity’ more broadly also refers to sharp changes of properties of single molecules, for example in protein folding, helix–coil transitions, or ligand binding, described in Chapter 26. In this chapter we focus on phase transitions.