YouTube, the world's most popular video sharing service, has been down for approximately 20 minutes as of this writing.
Details are still unknown as to why this has happened. As soon as more details are known, they will posted here.
As of now, YouTube's official support Twitter account is responding to users, saying that the development team is "looking into it."
Update (9:42 PM Eastern Daylight Time):
YouTube has posted an official update on the situation via Twitter:
Thanks for your reports about YouTube, YouTube TV and YouTube Music access issues. We're working on resolving this and will let you know once fixed. We apologize for any inconvenience this may cause and will keep you updated.