Every day thousands of telephone conversations are listened in on, e-mails intercepted and rooms bugged.
By Chris Summers
BBC News Online
The eavesdroppers include police officers, MI5 agents, private investigators, suspicious spouses and stalkers.
The targets range from terrorists, paedophiles and organised criminals to company executives and errant husbands.
Bugging technology is constantly updating
"Snooping" devices are doubtless a crucial tool in the fight against those planning dastardly crimes.
But bugs and phone taps can also threaten the human rights of innocent people.
"Spy" technology has come on leaps and bounds in the last 30 years and it is harder than ever to keep anything you say or write confidential.
Most people go through life entirely oblivious of the power of such technology.
But for those who have been targeted it can be a horrible experience which leaves a legacy of distrust and paranoia.
Telephone tapping and intrusive surveillance in the UK are covered by laws and guidelines.
Only the police and a handful of other law enforcement and intelligence agencies are allowed to use taps and each one has to be approved by the home secretary.
Up until 2000 warrants had to refer to specific addresses.
Intrusive surveillance (bugging)
Accessing financial, telephone and internet records
In 2001 they became "person specific", which meant each warrant must refer to a single target but can apply to numerous telephone numbers, for example home, business and mobile phones.
This meant the number of warrants issued fell from 2,080 in 2000 to 1,314 in 2001.
The figure did not include warrants issued on the grounds of national security to MI5 or military intelligence.
It also did not include any warrants issued in Northern Ireland.
Reg McKay, an investigative crime journalist in Scotland, believes the figures are misleading and says there are hundreds of "unofficial" taps being used by the police every day.
Mr McKay, who has been told by an electronic surveillance expert his own phone was "definitely" tapped, said: "I don't have a problem with the police tapping the phone if they suspect a major crime is being committed.
"What I do have a problem with is when it is done unofficially."
Rise in authorised phone tapping
1958: 95 warrants
Those who believe their phones are being monitored are in a Catch 22 situation.
The police are not obliged to confirm or deny their phones are being tapped, so they cannot take any legal action.
There is an independent watchdog - the Investigatory Powers Tribunal - but neither it, nor its predecessor, have upheld a single complaint over tapping in 18 years.
Who can apply for a phone tap warrant?
All UK police forces
Customs and Excise
Foreign police forces
The head of the civil rights group Liberty, John Wadham, admitted it would be counter-productive to allow all those targeted by the police to find out if their phones were tapped.
But he said: "There are many people who are identified by the police as targets but later realised to be completely innocent. In that case people should be told, at some time, that they were under surveillance."
Simon Davies, of the pressure group, Privacy International, said the vast majority of people who contacted his organisation thinking their phones were tapped were probably wrong.
He said: "It is very expensive to tap a phone. It costs about £1,000 an hour and 90% of the people who come to us would be of no interest to the police or security services.
"A lot of people hear clicking sounds on the line and think that means their phone is tapped. In reality the police usually 'clean' your line before tapping it to get the best sound quality so you are unlikely to hear clicks or hums," he said.
Patrick Mercer, a Conservative front bench spokesman on home affairs, told BBC News Online he supported the current system for police use of surveillance devices and was happy with the checks and balances in place.
A police bug caught Dion Griffin bragging about drug dealing
He said he knew of occasions, in Northern Ireland, when members of the security services had been dismissed for activating illegal phone taps.
"That was entirely right. We must control the security services just as we would a maverick individual and they should face the full force of the law if they fall foul of it," he said.
A Home Office spokesman pointed out that the Investigatory Powers Tribunal has the power to award compensation, make orders quashing or cancelling any warrant or authorisation, or make orders for the destruction of information or records.