The BBC is looking at the lives of five taxi drivers - who know the streets of the world's capitals better than anyone - and talking to them about their work, their city and the problems they face.
Here, the BBC's Chris Ledgard hitches a ride with the Black Cab drivers of London.
In theory, each of the 21,000 Black Cab drivers in Central London knows every street within 10km of Charing Cross.
The Black Cabs are one of the most distinctive features of London's streets
In practice that is impossible; but it takes a while to catch out Tom Willet, who has been doing the job for 30 years.
What makes these drivers so smart is the Knowledge - an exam which generally takes two to three years to pass.
Although there is talk of making the Knowledge more hi-tech, as it stands it has changed little over the years.
"They give you a book of runs, and the runs go from Point A to Point B, all over London," Tom explains.
"There were about 400 of these runs to do."
All this has to be done by motorbike, as it is not permitted to become a black cab driver until you have passed the Knowledge.
"You go from north London to south London, east to west, and on the way to pick up points - police stations, theatres. And you've got to know it. It's as simple as that," he says.
More passengers, less cash
London's Black Cabs are an essential part of the city's image.
Last month, the Prince of Wales visited a taxi shelter in central London to pay his own personal tribute to the work the cabbies do - and said that his father, the Duke of Edinburgh, often uses taxis to make his way around London unnoticed.
Prince Charles popped in for a chat with the cabbies at Hanover Square
These taxi shelters are dotted all over London - Victorian-origin dark greens cabins serving food and drink exclusively to cabbies. Tom's shelter is in Russell Square.
"You go in there and swap stories, talk nonsense and tell lies," he said.
"It's run by the famous Maureen, known throughout all north London. She's been involved in shelters for cab drivers for 25, 30 years."
Being a Black Cab driver is often a job for life. Those who have been in the business for many years say one of the biggest changes is the type of people who use the cabs - they are no longer just for the well-off.
"People you pick up now are far different to when I first started," one said.
"The ordinary man in the street never bothered with a cab. Now we pick people up with shopping at Sainsbury's."
However the rise in passengers has not meant a similar rise in income for the cab drivers.
"There are too many of us out now to earn a good living," Tom explained.
"Now, you've got minicabs, you've got motorcyclists, you've got twice as many buses, coaches.
"Over the years everybody's taken a slice of our pie."
The cab drivers are also known - as is the case the world over - for their strong opinions.
In particular, their anger is directed at the city's minicab drivers, who they see as unqualified rogue operators.
They admit the rivalry is fierce.
"Rivalry's a polite word," one cabbie said.
"They're everything we're not. We have to do the Knowledge. London's got the best taxi system in the world. We are renowned."
The mayor of London, Ken Livingstone, recently introduced a licensing system for the minicabs.
But the cabbies now want a clampdown on another group of passenger carriers - bicycle rickshaws.
The Licensed Taxi Drivers Association (LTDA) recently went to the High Court to try to force them to become licensed, but lost their case.
"They are a tragedy waiting to happen," argues one cabbie at Maureen's.
"They're not insured - they're just students who rent a rickshaw for the night and go out to try and earn a few bob. But all it's going to take is one bus, or one car, to crash into one."
Despite the frustrations, Tom says that the job does have it attractions too - such as working and travelling around such a varied, vibrant city.
In particular, he waxes lyrical when talking about the view across the famous Waterloo Bridge at night.
"It's absolutely beautiful," Tom says.
"I love it."