The internal pressure of a liquid, Pint, is the derivative of its internal energy with respect to the volume at a given temperature and represents an aspect of its cohesive energy density. The review summarizes the Pint of neat atomic, molecular and ionic liquids and liquid polymers at ambient conditions as well as of liquid metals and molten salts at elevated temperatures. The effects of temperature and pressure on the Pint of liquids, models dealing with it, and correlations with other quantities are discussed. The Pint of liquid mixtures leads to relations between the excess internal pressure and the self- and mutual interactions of the components. In the case of dilute aqueous solutions, the internal pressure increment over that of water relates to the water-structure-making and -breaking properties of the solutes, whether non-electrolytes or ions.