Because of their short coats, rotties generally don't have trouble with mats. However, if your rottie hasn't been brushed in a while or has gotten something stuck in his fur, use a detangling solution and a comb or mat breaker to gently remove the mats. If the mats are severe, take your rottie to a groomer to have the mats removed. Sometimes mats have to be shaved out with electronic clippers.