An ESN or electronic serial number is important to check if you are buying a used device. If the device you buy has been put on a black list for being stolen you will not be able to use it. Checking the ESN is a very easy process with many ways to do it. One way is to simply check with the service provider. Call the company the device is from and they can tell if the ESN is clean or not. However, there are also numerous websites created that you can check the ESN from for free. The options are limitless, so there is not reason to not check.