 There's a small deductible for Part B, and as with any deductible, it's paid annually when you incur covered Part B expenses. The deductible does not apply to every covered service, and generally speaking, Medicare Part B covers 80% of the cost of covered services. You, the beneficiary, are responsible for the other 20% after the deductible is paid, and there are rules around this premise as well. The provider must accept Medicare-approved amounts as full payment, and this practice is known as accepting assignment.