Medicare Supplement or Advantage: Who is Primary?

I seem to asked this one a lot and the answer I give is always the same. When you get on a Medicare Advantage Plan the Insurance Company becomes Primary. Usually this isn't a problem but it's also where the Doctor/Hospital Networks come in to play and some times you will need an authorization for certain procedures. With a Medicare Supplement, Medicare is still Primary. This is why there are no Networks with Medicare Supplement, you can go to any Doctor or Hospital that accepts Medicare.