The most significant of John Cardy’s contributions to theoretical physics concern the application of ideas of quantum field theory to statistical mechanics. In 1984, field theorists demonstrated that the property of conformal covariance imposes severe restrictions on behaviour of two-dimensional systems. John took this principle and turned it into a precise quantitative tool for examining two-dimensional criticality. As a by-product, he discovered a formula for black hole entropy in string theory. He single-handedly developed ideas of boundary conformal field theory, which has applications to quantum impurity problems as well as string theory. He discovered the Cardy formula for two-dimensional percolation, the study of which has led to two Fields Medals for other mathematicians. More recently, he developed ideas of quantum entanglement and non-equilibrium dynamics in many-body systems. He is a recipient of the Boltzmann Medal, the Onsager Prize and the Dirac Medals of both the ICTP and the IoP.