Five years ago hijackers flew planes into two towers of the World Trade Centre in New York.
Hours later the towers collapsed and about 3,000 people were killed.
Two other planes were also taken over. One crashed in Washington, the other in Pennsylvania.
Now the world is remembering the events of 11 September 2001, as well as looking towards the future for the site of the World Trade Centre, which has since been called Ground Zero.
Have the events since 2001 affected you? Have you been to New York since, or maybe you lived there when it happened.
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I felt really gutted for all them people who were involved and I wish for world peace.
Jamie, 9, Gorleston
When I heard the news about 9/11 I was scared to go to sleep.
Catherine, 13, Crawley
it is a very, very sad day to remember!
Charlie, 9, Warminster
My dad died in the attacks and I'm very sad about it. I hope that anyone who's lost someone is recovering slowly. I would like to say thanks to all my mates and my family for helping me get through this difficult time.
Katie, 13, New York
I think the people who done it are cold-hearted!
Lily, 11, Leeds
It is obviously very sad and also very memorable for me because my dad's friend was in the tower the day before it happened!
Mari, 11, Llangrannog
I remember the horror I had that day. I remember seeing the towers whenever I went to work with my dad, they were so big. My friend's dad was going to a meeting at the World Trade Centre on that day but luckily he was running late.
Olivia, 12, USA
I think it really sad. I just don't know what made terrorists do this terrible thing. May peace be with everyone who died.
Arushi, 11, London
I visited the World Trade Center in 1999 and then when I went back after the attacks I realised New York looks and just feels different. People now care more about others and are more patriotic. When things like this happen, and we all unite as one, it makes me proud to be American.
Shelley, 13, USA
I had actually been up one of the towers before they were destroyed. They were wonderful buildings and I think we should always remember the victims who were killed or hurt on that day.
Thomas, 10, Richmond
I live in lower Manhattan not far from where the towers were and I remember that day like it was yesterday. It changed America and the world forever. God bless all of the victims that were on the planes, in the Twin Towers, the Pentagon and on United Flight 93. United We Stand.
Jeff, 14, USA
I'm really angry. Those people were just trying to give Islam a bad name and I'm glad that there is nothing against Muslims here.
Fatimah, 12, London
I was only five-years-old when it happened and I have seen clips of everything and my heart reaches out to everyone affected by it.
Vanessa, 10, Banbury
Why would anyone do such a terrible thing? 3000 deaths but many more broken hearts.
Luke, 12, Dereham
I think violence is not the solution to any problem, it's just the root to another.
Sumaiyah, 12, Leicester
I don't remember much of it, I was only about seven, but I felt very angry as I don't understand why people would do such a thing. I mean, why would someone kill lots of people simply because they didn't like the USA?
Emily, 12, Newcastle
My sister was born the day before the attacks happened and I remember it very well.
Lucy, 13, Dublin
I visited Ground Zero about a year after the attacks and I felt really upset. I still wonder why someone would do something like that.
Sarah, 13, Liverpool
I feel that my school has absolutely no respect for those who died on 9/11 since there was no mention of the event throughout the whole day - we didn't have a whole school minute silence but a few of my friends and I had our own little memorial service.
Sally, 14, Kidderminster
I was in Year 3 when it happened but I remember everyone was crying and trying to explain to me what had happened.
What makes terrorists want to do this?
Gareth, 12, Cardiff
Why? What had the innocent people inside the towers done to deserve it? I hope the Freedom Towers will be a safer building and become the main feature of The Big Apple! I think they took a big bite out of the Big Apple with this attack and I feel for the people who lost there lives.
Ryan, 10, Manchester
It's disgusting! I was watching it on TV on September 11th and I still can't get over it. RIP.
Charlie, 13, Hertfordshire
I can't believe that somebody would do that. I still don't understand it.
Elley, 10, Liverpool
I was at the twin towers six weeks before 9/11. It was very amazing there and so I couldn't believe what happened.
Alex, 13, Fleet
I was disgusted that my school had no remembrance what so ever of this sad day.
Natalie, 14, Surrey
I am really upset even though I don't know anyone who died in the 9/11 attach. I just don't understand, WHY WOULD SOMEBODY DO THAT? My mum and I watched a programme about it, it was really bad. RIP to all those that have died.
Sophie, 12, Stratford
It's just terrible. We need peace in this world and it's horrific to think that someone would do that! It's cruel and evil. I just wish it all would stop!
Vicky, 11, Dorset
I was only seven years old when this happened so I was too young to really understand anything, but my mum doesn't want to go to New York again!
Sophina, 12, Surrey
I can't believe they would do that! We had a minute silence today and I prayed for all the people who died.
Rachael, 11, Wirral
I feel really sorry for the people who died.
Manal, 10, Glasgow
I feel devastated because my dad died on one of the planes. My school did a one minute silence, and during the assembly the head teacher said a few words about my dad.
Chantelle, 11, London
I went to New York in the summer holidays and I went to see Ground Zero and it was really upsetting seeing where it happened and seeing the names of people who died in the 9/11.
Amanda, 12, Scotland
Why are people so terrible? I feel for the people who suffered in 9/11, and for their families and friends, I hope they can rebuild their lives someday.
Charli, 12, Leicester
I went to the sight of the 9/11 attacks and it looked like they'd cleared a lot up but it's still hard to imagine what it was like for those who lost loved ones in 9/11.
Tara, 10, Cambridge
I'm going to New York on Friday and I'm hoping to visit Ground Zero.
Katie, 11, Bedford
I think it is a sad day for all the families that have lost a member of their family in that horrific disaster.
Daniel, 10, Belfast
It's disgusting! Why would anyone want to do that to another human being?
Kayla, 12, Jersey
I feel sorry for everyone who was effected by it. I am upset that some people have no respect for what happened.
Ellie, 13, Leeds
The 9/11 attacks were terrible and I feel so sorry for the families who lost loved ones.
Millie, 13, Cowes
It's a reminder that we need peace.
Zoe, 12, Trowbridge
What happened that day was dreadful and I still can't believe it really happened. My heart goes out to all the families that lost someone special that day.
Mia, 11, Kent
I feel very sorry for the people who got killed or injured in 9/11.
Becky, 14, Wolverhampton
It still upsets me to see what happened on 9/11. I think there is one question on everyone's lips - WHY?!
Emily, 12, Liverpool
We feel very sad for relatives who lost someone five years ago. We feel sorry for the people who lost their lives and we really hope this doesn't happen again.
Primary Class 5/6, Edinburgh
I think this attack was uncalled for and a disgrace to just to kill innocent people. When I first heard about it I was terrified.
George, 11, Derby
I think it was very sad and it has affected the world for ever.
Kosi, 10, Bristol
I knew a person who died that day. I still wonder why would somebody do that?
Rachel, 11, Penrith
That was a very sad moment. We had to have a minute silence in school today. Very sad!
Samantha, 12, Moreton
September 11th was very sad for me in 2001, and it still is in 2006.
Emir, 14, Turkey
I feel very sorry for all the people who lost their loved ones!
Aisha, 9, Chelmsford
I feel very sorry for those people that have lost someone close to them.
Charlie, 10, Chelmsford
I think that this will be a very sad time for some people but they must stay happy and not get too upset.
Harriet, 14, Newquay
I know seven people that were on one of the planes and I feel so terrible!
Sarah, 13, Manchester
I was there four months before the towers came down and I have just been looking at the pictures now and I am crying!
Eleanor, 11, Liverpool
I think it was a real tragedy that the twin towers got crashed into and such a shame because about 3,000 people died including loads of firemen.
Christopher, 10, Titchfield
It's scary to think that that kind of thing can still happen in this day and age. My thoughts go out to all people affected by this hideous act of terrorism.
Cara, 12, London
September 11th was very upsetting for me, I can't believe people would do that. I feel sorry for people who lost their loved ones.
Melissa, 13, London
Seeing Ground Zero up close really brings home the power of this site and makes you remember all the people who died on September 11. It makes you think about the conditions in the towers when they were hit and you think what it must have been like up there if the best option was to jump.
Jamie, 14, Surrey
I can't believe it's been five years. My thoughts are still with families who lost people and victims of the attack.
Katy, 13, Edinburgh
I think it is extremely sad and I don't know why someone would want to do something like that. I went to Ground Zero this year and the devastation is too bad for words.
Rachel, 12 Barrhead
When I remember what happened on that day, or I watch a documentary about it, I feel so sorry for the people who didn't get out.
RK, 13, Mill Hill
When it happened I was going to my new school and everyone around me was talking about the US being under attack. I didn't know much about what was going on but it is always going to a big part of everyone's lives now.
Alex, 14, USA
It's scary. I'm glad I wasn't in the towers when they collapsed, but it's too bad that other people were.
Maddie, 12, USA
We went on holiday to New York last year and visited Ground Zero. It was quite upsetting to really see where all the people died.
Jo, 10, Bournemouth