Gardeners in Britain are being told to plant more flowers which butterflies like, to try to stop their numbers going down.
Almost half of the 56 species of butterfly are under threat of extinction, according to experts.
Butterflies prefer certain flowers for their nectar and plants to eat when they are caterpillars.
Butterfly numbers have been falling for decades, and there are worries the speed is increasing.
President of wildlife organisation Butterfly Conservation, Sir David Attenborough, said: "If you change the environment to help butterflies, all sorts of other wildlife benefits too. Nature comes back to life."
In July they're organising their Big Butterfly Count, and they hope tens of thousands of people will take part to work out how butterflies numbers are changing.