A surprise medical bill is an unexpected bill, and one form involves bills for services received from a health care provider or facility that you did not know was out-of-network (e.g., had not negotiated a reimbursement rate with your insurance company) until you were billed. Your health insurance may not cover the entire out-of-network cost which could leave you owing the difference between the out of network provider’s bill and the amount your health insurance paid. This is known as “balance billing.” This bill could be for a service like anesthesiology or laboratory tests. You may not know that the provider or facility is out-of-network until you are billed.