Beware of the dog is a warning sign indicating that a dangerous dog is within.
Warning signs of this sort have been found in ancient Roman buildings such as the House of the Tragic Poet in Pompeii, which contains a mosaic with the caption "Cave canem". Some suppose that these warnings may sometimes have been intended to prevent visitors from stepping upon small, delicate dogs of the Italian greyhound type.[1]
Under English law, placing such a sign does not relieve the owner of responsibility for any harm which may come to people attacked by the dog.[2][3]
Such signs may be placed to deter burglary even if there is no dog.[4][5]