France classifies certain dog types into two legal categories under a 1999 ministerial order, and the distinction matters enormously if your dog resembles one of them. Category 1 ("chiens d'attaque") covers Staffordshire terrier, American Staffordshire terrier, mastiff and tosa types that aren't registered in an official pedigree book. Category 2 ("chiens de garde et de défense") covers the same breeds when they are pedigree-registered, plus the rottweiler. A vet can also classify any dog, regardless of breed, into one of these categories case by case if its build resembles the listed types.
The practical difference is stark. Category 1 dogs cannot be imported into France at all — acquiring, selling, breeding or bringing one into the country is prohibited outright, and they're barred from public transport, public places and any establishment open to the public, confined to being walked, muzzled and leashed, on public roads only. Category 2 dogs can enter and can use public transport and public places, but only muzzled and leashed, and only with a permis de détention (ownership permit) obtained from the local mairie — which requires a seven-hour training course, a vet's behavioural evaluation for dogs over eight months old, a sterilisation certificate (for category 1; not required for category 2), and proof of civil liability insurance.
If you're planning to bring a dog into France and you're not certain which category it might fall into — a build that resembles a listed type, even without the exact breed — it's worth getting a clear answer from a vet before you travel, not after. Operating without the required permit can bring a fine of up to €750, and failing to produce it when asked by a mairie or law enforcement officer can bring a separate fine of up to €450.