There are lots of reasons why you might need to find out the serial number of your iPhone, iPad or iPod touch. You might need it to get help with a technical issue from Apple support; you might want to check the status of your warranty; you may just want to find out which model of iPhone you've got.
Regardless of the reasons, we're here to help. This article covers six simple (in some cases extremely simple!) ways of finding that pesky serial number, and will help you even if your device won't turn on and you can't find the packaging. (However, bear in mind that for some support calls and the like, Apple employees will be able to work from the IMEI instead.)
Look on the back of the device
This is the easiest and most obvious method, but it doesn't work on every device. The serial number is printed on the back of the original iPhone, and on all iPads and iPod touches: for an iPhone 3G or later, you're out of luck.
On more recent iPhones, the serial number is easiest to find by opening the Settings app and going to General > About. Serial Number is the 11th entry on the list, and labelled as such.
(If you can't be bothered typing out the number you can tap and hold on it and select Copy when this pops up, then paste it into an email, Apple's support site etc. Or you can use Universal Clipboard if you need to get it on a Mac and you've got macOS Sierra or later.)
You can also find the serial number by plugging the device into a Mac or PC and opening iTunes; the serial number will be listed on its summary page.
If your iPhone or iPad is non-operational and the above strategies don't work, you can find the serial number on the original packaging, if you have it: it will be printed above one of the barcodes.
Not got the packaging? On the iPhone 3G, 3GS, 4 and 4S, the serial number is printed on the SIM tray.
Still no joy? If you've got an iPhone 5 or later, and it doesn't work well enough to get the serial number from Settings or iTunes, you can look in your Apple ID account settings instead.
Go to appleid.apple.com and sign in. Scroll down and you'll see a section labelled 'Devices'. Click the iPhone in question and you'll see the serial number and some other details.
You can do something similar if you've got another iOS device that's signed into the same Apple ID as the mystery iPhone: open Settings, then tap your name/face at the top, and scroll down to see your other devices and their details.