Apple's market cap is $413.06 billion versus $418.23 billion for Exxon.
Apple has been the world's most valuable company since August of 2011.
It comfortably held that position for over a year. But, as the world came to the realization that Apple wasn't going to forever grow its profits at an insane rate forever, the stock started to tank.
It's now more than 30% off its peak, giving Exxon its chance to regain the title as the most valuable company in the world.