Did you know that inside most tumours, there are areas with very low oxygen available, or maybe even no oxygen at all? And did you know these areas are very important for the cancer cells becoming more aggressive, harder to kill and easier to spread to other parts of the body? In this talk I will be discussing how this very interesting aspect of the tumour micro-environment is regulated, and how understanding the biology behind these changes and adaptations can help us to develop smarter and more specific ways to target cancer cells.