Plant secondary metabolism has multiple functions throughout the plant’s life cycle. These functions can be classified as mediators in the interaction of the plant with its environment, such as plant–insect, plant–microorganism and plant–plant interactions. Secondary metabolism also plays a role in plant reproduction, for example, in attracting pollinators and in male fertility. Secondary metabolites determine important aspects of human food quality (taste, colour and smell), and plant pigments are important for the diversity of ornamental plants and flowers. Moreover, several plant secondary metabolites are used for the production of medicines, dyes, insecticides, flavours and fragrances. Secondary metabolism is thus an interesting target for plant breeding.