In the first of a series, the BBC's Stephen Evans - who was in the World Trade Center at the time of the 11 September attacks - returns to New York to see how it has changed five years on.
There are obvious ways in which New York is different from the way it was on 10 September, 2001.
Downtown is transformed - the gaping gash of the World Trade Center remains largely unfilled.
The pit is acquiring the machinery which will be the working engine of the Freedom Tower, and there is a new underground station, but Ground Zero is still a wound. And that gives a different look to the whole city.
The twin towers were a landmark that could be seen all over Manhattan so if you emerged from the subway, you only had to look up and know which direction you were facing.
The gap when you look down 7th Avenue, for example, now feels like something is missing - which, of course, there is. The open sky seems like the absence of a limb or a pair of teeth knocked out.
New York has changed in quieter ways too.
Downtown was the great financial centre - Wall Street - arguably the financial capital of the world. It still is, though many of the people who worked there now work elsewhere.
The attacks on 11 September pushed the big finance houses out from what had been often old, inconvenient offices, and many have not moved back.
There has been some regeneration, but often by people in homes rather than companies in offices. The mix of the region has changed, with more residential accommodation, partly encouraged by the lower rents on offer after the attacks.
Some people were uneasy about living Downtown - there was a feeling that the place had the smell of death to it, literally. The ghosts were too obvious.
And the people have also changed.
Talk to anybody and they have got a story to tell of what they were doing on the day. How can you witness two planes being flown into towers of innocent people without thinking a lot about it?
Take David Handschuh. He is still a photographer on the Daily News but now he confines his pictures to food - something he says makes people happy - rather than to the car crashes and crime which were his daily diet before.
David was probably the first photographer on the scene on 11 September, arriving before the second plane went in. He witnessed at close view through a lens the people jumping and heard them landing - and all before getting trapped under the falling south tower to be pulled out, badly injured, by fire-fighters.
He has changed. To drive with him round the city now is to experience a great external calm.
A lorry cuts him up and he smiles saying: "Go ahead. You're bigger than I am."
"Having a good day with my kids is the most precious gift that I could have in the entire world," he says.
Children were especially affected. Picture: Ryan Sweeney, courtesy NYU Child Study Center
"Waking up the next morning and looking out the window and whether it's snowing or raining or storming or brutally hot or freezing cold, it's another day that we get to experience things on this Earth."
For him, there has been a long and painful process of psychological recovery - it has been about mastering images which haunt him.
"As a kid, you worried about the monsters under your bed, and you'd hear a noise and it would spook you a little bit and your mom or dad would come in and say 'everything's okay, don't worry'. You check with a flashlight under the bed and there's no monsters hiding underneath there.
"Well, I got a bunch of little gremlins.
"I think anybody who is in New York, or who lost somebody or who paid witness that day, has a bunch of little gremlins under their bed, and every once in a while those gremlins leap out and they taunt you and they bite you and they want to play with you.
"So you play with them and then you put them back under your bed and maybe it's five minutes, maybe it's five days, maybe it's five months till they come back out and play, but you've got to confront your gremlins and then say 'You know what, folks, it's time to move on, I'll see you in six months'."
Children, too, had to cope with witnessing terror like the sight and - perhaps more importantly - the sound of a person falling to his or her death.
Robin Goodman is a psychologist who worked with children. Many of them drew pictures of horror - two planes crashing into two towers, fire, Osama Bin Laden snatching the towers and stuffing them into his mouth.
"You had kids building towers with Lego and crashing them. That's not uncommon when you've been through something traumatic, re-enacting it and re-experiencing it. It's a way to get mastery over it.
"If kids keep doing that and doing that, and don't get relief, it can be a problem".
Diane Horning wants a final resting place for her son
It is a matter of coping with memory and loss.
For Diane Horning, the problem is different. She lost her 26-year-old son, Matthew.
Unlike the relatives of some 1,150 people who were never identified, Diane has a small amount of Matthew's remains.
But she believes the rest of his body to be in the fine siftings of material from Ground Zero that are on the Fresh Kills landfill site on Staten Island.
Five years on she is still tirelessly campaigning to have these "fines" moved and buried "somewhere decent".
"We're not even asking them to DNA test them. We've said 'give us a cemetery that is a common grave where we can put a marker for everyone, even if we don't have individualized spots for each person'."
Remains are still being found at the World Trade Center site, and Diane says it's important to relatives. It is important, too, to treat those remains with respect.
"When I see Staten Island, when I see the garbage dump - I realise that none of us can have the solace of a resting place, unless it's a dignified resting place," she says.