ANPR cameras photograph passing vehicles, extract the number plate using optical character recognition, and check it in real time against police databases (stolen vehicles, vehicles linked to a suspect or crime, uninsured or untaxed vehicles) and DVLA records. A read that matches a flagged plate generates an alert; a read that doesn't match is still normally retained, since the investigative value of ANPR often comes from reconstructing a vehicle's movements after the fact, not just catching a live match.

In the UK, the National ANPR Service processes tens of millions of reads a day across a shared national camera network, with retention periods and access controls set nationally rather than force by force. In the US, ANPR (more often called ALPR, automatic licence plate reader, there) is largely a commercial product bought directly by individual police departments and cities, with far less national standardisation of retention or access; Flock Safety is the dominant US vendor and has its own entry here given how much it differs in practice — privately run, subscription-based, and city-by-city in its governance, rather than a single national system.