A differential equation is an equation that contains an unknown function, which we need to solve for, and its derivatives. Technically they are ordinary differential equations (ODEs) since they contain ordinary derivatives as opposed to partial derivatives. An equation that contains partial derivatives is called a partial differential equation (PDE). In this module we shall only consider ordinary differential equations. Differential equations are extremely important in science and engineering as they can mathematically describe physical processes such as current flow in electrical systems, motion of mechanical systems, fluid flow, chemical reactions, population dynamics, the spread of infectious diseases, and many other natural phenomena.