A huge hedgehog hunt called Hogwatch has begun, to find out how many of the prickly creatures there are in the UK.
The People's Trust for Endangered Species is asking people to report sightings of the animals as they are worried hedgehog numbers are falling.
Some experts fear the number of hedgehogs in the UK has dropped by as much as 20% over the last four years.
Hedgehogs are thought to be under threat from farming and the loss of their habitat in towns.
Thousands of people are expected to take part in the online survey, which is being organised with help from the British Hedgehog Preservation Society.
Hogwatch co-ordinator Paul Bright, of Royal Holloway, University of London, said: "It is vital to know where hedgehogs are still present and determine why their numbers appear to be falling.
"Hedgehogs have been around for 20 million years. We want to ensure they will be around in the next century."
The information will be used to put together a UK hedgehog map, which will show exactly where the creatures are living.