In recent years, it has become increasingly clear that successful cancer therapy must be tailored to the individual needs of people suffering from cancer. In this talk, we trace the question of what the biological basis of cancer is and how we can apply this knowledge to develop tailored treatments for each individual. We look at how and why there has been an impressive increase in the amount of data we can now obtain from a cancer patient’s tissue sample. We think about the role of machine learning and artificial intelligence in cancer medicine. And we consider what weather forecasts have to do with therapy decisions in oncology.