It is perfectly normal thing for them to do when they want to give warning and call for assistance. Persistent barking, however, can drive owners and neighbors up the wall. Some breeds are naturally more vociferous than others, so if you are looking for a relatively quiet dog, don't by one of the guarding breeds. If you do have a guarding breed, don't give it the run of your garden when you go to work, otherwise you will come to find some irate neighbours brandishing a court order. Having accepted that the basic function of barking is to give warning, a lot of dogs bark for other reasons. The barking as an attention-seeking behaviour; they bark out of excitement; some highly excitable types bark at every little noise, and some dogs bark for the sheer hell of it. If you have a problem barker, it is sensible to try and establish what triggers it off and whether this is the area that you should be controlling.