Cancer cells are characterised by altered metabolism compared to healthy tissue. Ninety years ago, the German medical doctor and biochemist Otto Warburg discovered that tumours--when compared to healthy tissue--convert considerably more glucose into lactate, regardless of oxygen presence. This phenomenon is known as the Warburg effect; it is a hallmark of most cancer types and can be well understood by the process of somatic evolution. The Warburg effect explains the significance of the PET scan and may offer opportunities for new treatments of cancer.