Part B medical insurance helps pay for some services and products not covered by Part A, generally on an outpatient basis (but also when on an unadmitted observation status in a hospital). Part B is optional. It is often deferred if the beneficiary or his/her spouse is still working and has group health coverage through that employer. There is a lifetime penalty (10% per year on the premium) imposed for not enrolling in Part B when first eligible.

Part B coverage begins once a patient meets his or her deductible ($183 for 2017), then typically Medicare covers 80% of the RUC-set rate for approved services, while the remaining 20% is the responsibility of the patient, either directly or indirectly by private group retiree or Medigap insurance.

Part B coverage includes outpatient physician services, visiting nurse, and other services such as x-rays, laboratory and diagnostic tests, influenza and pneumonia vaccinations, blood transfusions, renal dialysis, outpatient hospital procedures, limited ambulance transportation, immunosuppressive drugs for organ transplant recipients, chemotherapy, hormonal treatments such as Lupron, and other outpatient medical treatments administered in a doctor's office. It also includes chiropractic care. Medication administration is covered under Part B if it is administered by the physician during an office visit.

Part B also helps with durable medical equipment (DME), including but not limited to canes, walkers, lift chairs, wheelchairs, and mobility scooters for those with mobility impairments. Prosthetic devices such as artificial limbs and breast prosthesis following mastectomy, as well as one pair of eyeglasses following cataract surgery, and oxygen for home use are also covered.

Complex rules control Part B benefits, and periodically issued advisories describe coverage criteria. On the national level these advisories are issued by CMS, and are known as National Coverage Determinations (NCD). Local Coverage Determinations (LCD) apply within the multi-state area managed by a specific regional Medicare Part B contractor (which is an insurance company), and Local Medical Review Policies (LMRP) were superseded by LCDs in 2003. Coverage information is also located in the CMS Internet-Only Manuals (IOM), the Code of Federal Regulations (CFR), the Social Security Act, and the Federal Register.