Naturally fluorescent proteins have revolutionized biology by enabling what was formerly invisible to be seen clearly. These proteins have allowed us to visualize, in real time, important aspects of cancer in living animals, including tumour cell mobility, invasion, metastasis and angiogenesis. These multicoloured proteins have allowed the colour-coding of cancer cells growing in vivo and enabled the distinction of host from tumour with single-cell resolution. Visualization of many aspects of cancer initiation and progression in vivo should be possible with fluorescent proteins.