A new approach to medical imaging uses light to generate images both of newborn babies’ brains and of breast cancer. Blood absorbs light strongly and its colour depends on how much oxygen it is carrying, so imaging using light provides a direct measurement of both the volume of blood and its oxygen content. We have used this technique, called optical tomography, to image the blood supply to the brains of both ill and healthy premature babies, and the increased blood flow to cancer in the breast. Optical tomography provides relevant clinical information which may not otherwise be available.