Heading into retirement brings a slew of new topics to grapple with, and one of the most maddening may be Medicare. Figuring out when to enroll in Medicare and which parts to enroll in can be daunting even for the savviest retirees. There’s Part A, Part B, Part D, medigap plans, Medicare Advantage plans and so on. And what the heck is a doughnut hole, anyway? To help you wade into the waters of this complicated federal health insurance program for retirement-age Americans, here are 11 essential things you must know about Medicare.