Revenue protection inspector (RPI) or revenue protection officer (RPO) is the job title given to staff who patrol different forms of public transport issuing penalty fares to passengers who travel without a valid ticket or without the correct ticket (e.g. travelling where the ticket is not valid or a commuter using a concession ticket without possessing appropriate evidence of entitlement to concession). Bus inspectors also check whether the bus is running to time and whether the driver/conductor is collecting fares correctly, and attempt to maintain headways.
These titles are principally used in the United Kingdom and Australia. In the United States, and Canada, fare inspectors are transit police officers who audit transit passengers for proper payment of fare; in addition in some cases, regular police officers are authorized to conduct fare inspections on some systems.