I've got an "invisible fence" added to our real fence because they kept climbing over and getting out. One of my cats, Baby, got over the fence and got killed by a dog. Now if they get within three feet of it, they get a mild electric shock and back off. I haven't seen any of them venture near the fence since the day after it was installed. I asked my vet before I had it installed--he said it was very mild and it was much more humane than getting hit by a car or ripped to shreds by another animal.