Indirect evidence forms the basis of our knowledge of the natural history of human breast cancer. It is assumed that breast cancer starts by subtle molecular changes within a cell, called the induction phase, leading to the creation of a malignant cell. This phase is followed by multiplication and progressive growth of the cancer cell, most often leading to disseminated disease and to the death of the patient. From clinical observation it can be concluded that the range in growth rates of breast cancer is wide, and probably also the moment and pattern of metastatic dissemination. Some cancers disseminate early, but others may disseminate late or not at all. From serial mammographies it appeared that growth rates, expressed as tumour volume doubling times, ranged from about two months to several years.