Emergency medical transportation services have some different characteristics than other patient services that make billing for the services more difficult than for other providers. Payers have been less than systematic in their definition of medical necessity, a criterion that is required in order to justify reimbursement. As a result, documentation of an ambulance trip and appropriate coding of the activities carried out during the course of the trip are key elements in determining whether or not the service will be reimbursed. This article examines the many dilemmas associated with this issue.