Cells have to organize their complex biochemistry to regulate their metabolism and respond to changes in the environment. Traditionally, intracellular organization has been associated with compartments that are surrounded by lipid membranes. However, in recent years, phase transitions have emerged as a novel form of cellular organization. Phase transition is a physical process whereby a substance changes from one physical state to another. Examples are provided by the freezing of water into ice (liquid to solid) or the heating of water to generate water vapor (liquid to gas). In this Primer, Alberti introduces the emerging topic of biological phase transitions and discusses how cells use phase transitions to regulate biological functions and activities in order to increase their fitness and chances of survival.