Section 1 of this unit looks at the wave equation, in the context of modelling the vibrations of a taut string (such as guitar string). Another partial differential equation, the diffusion equation, is developed in Section 2. In this unit we deal only with second-order partial differential equations. Sections 3, 4 and 5 are concerned with solving examples of the wave equation and the diffusion equation. Section 5 shows how problems involving these equations can readily be solved with the aid of the computer.