Reciprocity - that is, recognition of one state's EMT certification being valid in another state - between states is somewhat limited, and after 30 years of operation by the National Registry of Emergency Medical Technicians, only about 40 states provide unlimited recognition of the NREMT certifications. In reality, there are at least 40 types of certification for EMS personnel within the United States, and many of these are recognized by no more than a single state. This creates significant challenges for the career mobility of many EMS providers, as they must often re-sit certification examinations each time they move from one state to another.