From today, sheep will live on Hampstead Heath in London for the first time in 60 years.
A small flock of five animals will be released into the huge park in the north of the city.
Two types of sheep are part of the project - Norfolk Horn and Oxford Down.
While in the park, the sheep will nibble down the grass as a trial to see if this will work as an eco-friendly way of maintaining the park.
Looking after land through animal grazing like this is a tradition that dates back for centuries.
Fencing has been installed around the area to protect sheep.
The idea has come from the City of London Corporation, which has said: "Grazing is known to play a major role in boosting species-rich wildlife habitats."
If the one-week trial is a success, the corporation said that more sheep could be put into different areas of the park.