I am not a qualified behaviourist but I have read many books about dog behaviour & training. It is very difficult to give advise over a forum without seeing the dog in person & assessing the situation as a whole, including body language. Can you give me a little more information / detail ? When is your dog agressive ? Is it with other dogs, resourcing guarding, towards strangers ? What do you when your dog is aggressive ? What have you done to try remedy the situation to date ?