One in three people develop cancer during their lives and 10–20 per cent of human cancers are linked to viruses. ‘Tumour viruses’ shows how viral oncogenes have been instrumental in uncovering the molecular mechanisms involved in cancer development. Tumours develop when a single cell in an organism is released from the usual constraints that regulate its growth and then replicates unchecked. This rogue cell then produces a mass of similar cells, forming a tumour (or cancer) that invades the surrounding tissues and may spread from its original site. The tumour-causing behaviour of oncogenic retroviruses, human T lymphotropic virus, herpesviruses, hepatitis viruses, and papilloma viruses are all considered.