As you may have heard, Pokémon GO is currently in the process of its worldwide rollout and has made its way to many new European and Asian cities this week. If Pokémon GO is already available in your country but you are having trouble downloading the application, it is highly likely that your phone is not (yet) compatible with the game. Here's a rundown of all the devices and operating systems currently compatible with Pokémon GO.
On a very basic level, Pokémon GO requires a good internet connection (preferably WiFi or 4G/LTE with a data package) and an active GPS system. It is also recommended that your device have at least 2 GB of RAM for smooth and uninterrupted use.
Pokémon GO is compatible with any Android devices running version 4.4 or later as well as iPhone devices running iOS 8 or later.
If your device fulfils the above system requirements, a very easy way of finding out if your specific device can play Pokémon GO is downloading it from your respective App Store. Either the Google Play Store, or the Apple App Store.