Apple today unveiled its third-gen Apple Watch, which offers a cellular connection for the first time.
Go for a run, take the dog for a walk, or head to the store wearing your Apple Watch Series 3, and there's no need to bring the iPhone along. You will, however, need to own an iPhone for the watch to work— specifically, an iPhone 6 or later with iOS 11 or later.
The iPhone and watch must also be on the same carrier. Just add your smartwatch to your mobile plan; it will share the same number as your iPhone. The big four mobile carriers will sell the Series 3; plan pricing was not revealed, but it will reportedly set you back about $10 per month. Apple will also sell a non-LTE GPS version of the Series 3.
The waterproof Series 3 device runs a dual-core S3 processor and custom W2 wireless chip, which should provide all-day battery life up to 18 hours, Apple COO Jeff Williams said at today's press event. A barometric altimeter will measure elevation and help track activity, flights climbed, and outdoor workouts. The Series 3 is also water-resistant up to 50 meters.
The device's software also gets an upgrade to watchOS 4, which includes an updated Heart Rate app that can, for example, notify you if you have an elevated heart rate but you don't appear to be active. 
Siri will also be more "proactive," according to Apple, and show you things you need to know. The Workout app also gets a makeover, and GymKit will let you connect your watch to supported gym equipment. In the fall, you can stream Apple Music on the cellular + GPS Series 3—and listen via your AirPods, of course.
But "the biggest challenge of all was adding cellular," Williams said. "Our little watch is already packed," but for cellular, you have to add antennas, radios, and a SIM card, among other things. "If you don't get it right, it looks like a house arrest bracelet and you won't want to wear it," Williams quipped.
Rather than add an antenna, the Series 3 display is a multi-frequency antenna for LTE and UMTS. There's also an electronic SIM ("even a nano SIM is too big," Williams said), so Series 3 ends up being the same size is Series 2.
Apple Watch Series 3 with cellular + GPS starts at $399. The GPS version of Series 3 will run you $329, while the older Series 1 now starts at $249. Pre-orders begin on Sept. 15 and the devices arrive Sept. 22.
As usual, Apple will also offer new bands, including pricey ones from Hermes, which start at $1,149. A new gray ceramic Apple Watch Edition will be $1,299.