A stolen smartphone can ruin anyone's day, though the latest built-in anti-theft software can ruin the thief's too. Here's how to enable your phone's kill switch - and why you should.
For the past few years Google and Apple have included theft-deterring software as a requirement of US law. Most of it is packaged in the form of activation locks and remote device erasure software. It's already in your phone, and it's incredibly easy to make sure it's actually on.
iOS: Find My iPhone
Every iPhone associated with an iCloud account features both Find My iPhone, which lets you track your device's last known location, and Activation Lock, a feature requiring you to enter the associated username and password on the device.
If you're selling your phone or giving the device to a friend, you can turn Activation Lock off after both turning Find My iPhone off and removing your account from the device itself. If your phone is stolen, Activation Lock will stop ne'er-do-wells from associating the phone with their own account, making it a useless slab of glass and aluminium.
You should also enable Find My iPhone. In iOS, visit Settings > Apple ID, then select the device you're using. From there you can enable or disable Find My iPhone, or view your currently lost device. You'll also need to enable the Find My iPhone app's location access. Turn it on in Settings > Privacy > Location Services.
Android: Find My Device
Find My Device is Google's device tracking software, accessible in your phone's settings page. Open Settings > Google > Security, then turn on "Remotely locate this device" and "Allow remote lock and erase". From there, you can locate your device from any browser you're signed into. Just Google "find my phone" and if your Find My Device software is enabled, it should display its latest location.
You'll also need to unhide your device from Google Play. Visit Google Play's settings page and toggle your device's visibility there.
If your thief happens to factory reset the device in order to claim it for their own, they will have to prove they're you. According to Google, "You can sign in with any Google Account previously added and synced to the device as an account or user, but not as a guest. If you can't give this information during setup, you won't be able to use the device at all after factory reset."