I must politely disagree with this. A comprehensive motion/IR/glass-break sensor array tied into a full-blown alarm system with video cameras, audio recording, a localized, in-the-home battery backup, AND a 3rd party monitoring connection .... is the best warning system. Such a setup costs about as much as a pure-bred work dog (like Dobermans or German Shepherds) costs from a reputable breeder. However, they consume less in energy than such dogs consume in food. Insurance will give you a discount for having them ... but not for having a dog. Such systems don't have to be kenneled when you're gone and, in fact, continue to do their jobs while you're out. They also don't have to be walked or taken outside and they don't crap on your rug or your lawn; housebreaking is not an issue. You also don't need to bathe them to keep them from smelling bad... For about 85 bucks you can even have the illusion of a dog, too, with none of the hassles -- as such systems have options for barking dog emulators that bark when the system detects something. Given this, the only good reason to own a dog is because you want one; a high end alarm system does more (like calling the police, the fire department, etc ... even when you're not home) than a dog, costs less than a dog over the long run, and entails many less hassles.