They're all around us, in the soil, our bodies and the air, but are often too small to be seen with the naked eye.
They provide medicines and food but also wreak havoc by causing plant and animal diseases.
According to the first big assessment of the state of the world's fungi, the fungal kingdom is vital to life on Earth.
Yet, more than 90 per cent of the estimated 3.8 million fungi in the world are currently unknown to science.
"It's such an interesting set of organisms and we really know so little about them," says Prof Kathy Willis, director of science at the Royal Botanic Gardens, Kew, which led the report.
"They're really weird organisms with the most bizarre life cycle. And yet when you understand their role in the Earth's ecosystem, you realise that they underpin life on Earth."
Many people are familiar with edible mushrooms or the mould behind penicillin. But fungi have a range of vital roles, from helping plants draw water and nutrients from the soil to medicines that can lower blood cholesterol or enable organ transplants.
Fungi also hold promise for breaking down plastics and generating new types of biofuels. But they have a darker side: devastating trees, crops and other plants across the world, and wiping out animals such as amphibians.
Jekyll and Hyde
Dr Ester Gaya, who leads a research project at Kew exploring the diversity and evolution of the world's fungi, says fungi are a bit like Dr Jekyll and Mr Hyde.
"They can be good and also bad at the same time," she says. "The same fungus, it can be seen as a detrimental thing - it can be bad - but also can have a lot of potential and have a lot of solutions."
The report sheds light on a number of gaps in our knowledge of a group of organisms that may hold the answers to food security. The fungal kingdom contains some of the most damaging crop pathogens. But fungi also recycle nutrients and play a role in the regulation of carbon dioxide levels.
"We ignore fungi at our peril," says Prof Willis. "This is a kingdom we have to start to take seriously, especially with climate change and all the other challenges that we're being faced with."
Fascinating facts about fungi
- Fungi are in a kingdom of their own but are closer to animals than plants
- They have chemicals in their cell walls shared with lobsters and crabs
- A fungus has been discovered capable of breaking down plastics in weeks rather than years
- There is evidence to suggest that yeasts - a type of fungus - were being used to produce the alcoholic drink mead as long ago as 9,000 years ago
- At least 350 species are consumed as foods including truffles, which can sell for thousands of dollars apiece, quorn, and those in marmite and cheese
- Plastic car parts, synthetic rubber and lego are made using itaconic acid derived from a fungus
- 216 species of fungi are thought to be hallucinogenic
- Fungi are being used to turn crop waste into bioethanol
- Products made from fungi can be used as replacements for polystyrene foam, leather and building materials
- DNA studies show that there are thousands of different fungi in a single sample of soil, many of which are unknown and hidden - so-called "dark taxa"