The successful design of optical imaging systems requires many skills. In this paper we are considering 'lens design,' the design of the refracting (or reflecting, or sometimes diffracting) surfaces in an optical imaging system, to give the best possible image quality. To be effective in lens design, it is necessary to understand geometrical optics, aberration theory, physical optics and image formation, and to be able to use optical design software and to know what optical systems have been used successfully in the past. In addition some understanding of optical testing and the properties of sources and detectors is also necessary. After teaching lens design for over 30 years, I have come to the conclusion that, while other topics are clearly important, the central subject, that must be well understood, is aberration theory. Without a good understanding of aberration theory the designer will simply try one 'trail-and-error' solution after another, possibly even using a computer program to generate these solutions automatically. In our courses, we teach students about Seidel aberrations, and about other topics which help understanding. We concentrate on explaining which aberrations are introduced by individual surfaces, or by individual groups of surfaces: from this the designer should appreciate which types of design have the potential to give good performance, and can use lens design software intelligently.