Planet Earth is home to 8.7 million species of animals and plants, scientists have worked out.
They think this is the best estimate that's ever been made. Previous guesses have ranged from 3 million to 100 million.
But most species haven't even been found yet - and cataloguing them all could take more than 1,000 years!
Experts warn that many species will become extinct before they get to be identified.
For hundreds of years, scientists have organised similar species into groups to create a giant "family tree".
The team used this method of grouping species together to calculate how many unknown animals and plants there are.
Only about 1.2 million species have been officially described so far.
Lead scientist Dr Camilo Mora said: "The question of how many species exist has intrigued scientists for centuries.
"The answer... is particularly important now because a host of human activities and influences are accelerating the rate of extinctions."