They're eating too much, ruining crops and woodlands.
They're also wandering on to the roads, causing accidents.
The report - by conservation groups the WWF and the Royal Society For The Protection Of Birds - has led to calls to control the deer population.
This could include culling, which involves killing some of them off.
But not everyone agrees that culling is the answer.
So, there will be no changes until more research is carried out.