Smoke alarm / carbon monoxide detector for the 21st century. It connects to your Wifi (also to other Nest Protects within 50 feet) and smart phone app for peace of mind when your away from home. The app will also let you know if the battery is dying. If you don't have an app the unit itself will glow yellow to let you know. Genius! It also speaks in a clear voice. It will tell you what the potential problem is. If it senses that it's an immediate emergency, an alarm will sound. And will alert other Nest Protects in the home of what and where the problem is occurring. Genius! Plus it gives you a nightly glow of green to let you know it is functioning ok. And it's a night light in the dark. Genius! Not only that but it makes turning of the alarm as simple as waving your hand in front of it. Genius! I only gave it a 9 because it has yet to be tested in the midst of an emergency at my house. But I am confident it will perform as advertised.