Since we live in a world where interesting things happen both in time and space, the description of many natural phenomena requires the study of functions that depend on multiple variables. These laws can be expressed as partial differential equations. Just like ordinary differential equations, these equations are the natural mathematical tool for modelling because they express, locally, basic conservation laws such as the conservation of mass and energy. ‘What’s the frequency, Kenneth? Waves, quakes, and solitons’ introduces the wonder of partial differential equations by considering two generic spatiotemporal behaviours: wave propagation through linear waves on a string and earthquakes, and non-linear waves and the discovery of solitons, which behave like discrete particles.