evidence: Both the gut and the tumour microbiome are now established as crucial regulators of cancer phenotypes and have been implicated in cancer initiation, progression and therapy response. Although the role of bacteria in these processes is beginning to be unravelled, the relevance of fungi is only just emerging. In this Viewpoint, we asked experts to discuss the current knowledge on the mycobiome–cancer connection and share their opinion on how to best solve open questions. [...] Galloway-Peña, J., Iliev, I.D. & McAllister, F. Fungi in cancer.