‘What is a fungus?’ explores the evolutionary origins of fungi and how they differ from plants and animals. The fungi are united by three principal characteristics: they are eukaryotes, which feed by absorption, and reproduce by forming spores. Fungi are more closely related to animals than they are to plants; fungi and animals both belong to the Opisthokonta taxonomic supergroup. The emergence of the fungi as a distinctive group of organisms is estimated to have happened between 760 million years ago and one billion years ago. The structure, growth, and physiology of the fungal cell are also described.