People who commit alcohol-related crime will be made to wear ankle tags that monitor their drinking under a new scheme to be trialled in four London boroughs.
But how do they work?
The "sobriety tags" will be worn by offenders 24 hours a day.
They will measure a person's sweat and air emissions from the skin every 30 minutes to see if it contains alcohol.
The device appears to be tamper-proof with multiple sensors ready to detect obstructions and removals, according to one manufacturer.
What happens if a positive trace is found?
An alert will be sent to the offender's probation officer.
The person can then be recalled to court, where they could be fined or be re-sentenced.
Has the concept been used before?
A similar scheme was successful in the US.
Actress Lindsay Lohan was forced to wear the bracelet after breaching her bail conditions for her drink-driving arrest.
Former White House Drugs advisor Professor Keith Humphreys says the idea has had a "transformative effect" in the US.
Mayor of London Boris Johnson hopes the success of the scheme can be replicated to tackle "alcohol-fuelled criminal behaviour" in the capital.
If the scheme is effective it could be rolled out across the UK.