Cancer researchers require accurate diagnoses for the samples, cell lines, patients or populations that they study. These diagnoses are underpinned by an internationally accepted taxonomy – the World Health Organization Classification of Tumours. This is still largely based on the histopathological examination of biopsy specimens, but increasingly also molecular methods and radiological examination of patients. Classifications evolve as new evidence arises, and for tumours that evidence is available in a quantity that is both remarkable and daunting. Evaluating this deluge of new information and incorporating it into the World Health Organization Classification of Tumours is now the responsibility of an editorial board, and up to 200 editors and authors work on each system to update it within the new 5th edition. Just as cancer researchers depend on the classification for diagnoses, so too the classification depends on the generation of high-quality, trustworthy data by cancer researchers. It is not just a case of quantity but quality too. Scientific fraud is thankfully rare, but high-profile cases are damaging and standards need to improve, not least to ensure that accurate information enters the classification.