More than 100 pirate radio stations are suspected to be operating in the UK today.
And after police in Sunderland carried out a house raid on a suspected pirate radio station in the city, we've taken a look at what makes such stations illegal and how those convicted of running them could be punished.
What makes a radio broadcast or station "pirate"?
A "pirate" radio station is one which broadcasts without a valid licence.
According to broadcast regulator Ofcom, illegal stations and broadcasters can often use equipment which may interfere or disrupt the communications of critical services like air traffic control.
In some cases, so-called "pirate" stations are considered legal where the signal is transmitted, but illegal where the signal is received. Broadcasts can also be classed as pirate depending on their content, the format of their transmission and the transmit power.
Pirate radio first rose to popularity in the UK in the 1960s thanks to the growing demand for pop and rock music, which was not broadcast on legal BBC Radio services.
Anyone involved in illegal broadcasting is committing a criminal offence.
What are the punishments?
Those involved in illegal broadcasting could face up to two years in prison or an unlimited fine under the Wireless Telegraphy Act 2006. You could be classed as committing an offence if you know, or have reasonable cause to believe that unauthorised broadcasts are being made, and:
* Keep a station/apparatus available for its use
* Allow your premises to be used
* Provide content
* Otherwise enable the broadcasting, including managing and rendering any service that will facilitate the operation
It is also an offence for someone to knowingly permit the use of their premises for illegal broadcasting, and not taking reasonable steps to prevent it.
How do I broadcast without breaking the law?
Ofcom can issue both FM and AM community radio licences which enable stations to broadcast legally in their local area.
More than 230 of these stations are currently broadcasting in the UK.
A station can still apply for a licence even if it has previously broadcast unlawfully - but if someone has been convicted of unlawfully broadcasting they cannot hold a radio licence for five years.
Will I know anyone from a pirate radio station?
John Peel began his radio career with pirate station Radio London in 1967. Veteran DJ Tony Blackburn also worked with Radio London, and Radio Caroline, which was founded in 1964 and broadcast from international waters using five different ships for more than 25 years.
Former pirate radio stations which are now licensed include Kiss 100 London, Sunrise Radio and XFM.