Update, July 17, 2016: After being coaxed back to life Saturday evening, Pokémon Go's server issues seem to be continuing into Sunday, as newly-launched countries get stuck into the game.
It appears as though the Pokémon Go login servers are experiencing a bit of trouble, preventing users from being able to log in and play the game. Have you noticed any issues, or are you still out there catching them all? Niantic has been adding support for gameplay in several new countries over the recent days, and odds are that the servers are just simply overloaded at this point.
Server issues are never fun, but if you are experiencing them you are not alone. If you notice the app slowing down, or the inability to complete a login on your phone, just be patient while the teams behind the scene sort this out. Be sure to let us know if you are seeing any issues, or if you are able to log in and play as normal.