September 11, 2001, is a day most people will never forget. It is the day the world changed forever.
Most people you speak to will be able to tell you exactly where they were and what they were doing when those two planes hit the twin towers in New York.
In terms of an act of terror, the Boston marathon bombing is not quite in the same league as the World Trade Centre attack, but it is still very sad and unsettling.
What makes this latest terror attack - the first on American soil since 2001 - is that it is the sort of attack event organisers the world over have feared. How many times have we heard, in the lead-up to a big sporting event, organisers say that they have beefed up security to prevent an attack happening?
As far as world-renowned events go, the Boston Marathon is right up there. As a marathon, it as well known as the London Marathon and the New York Marathon. The most horrifying aspect of this attack is that it happened at a sporting event where people were out in large numbers enjoying themselves.
At the time of writing, US authorities still did not know who was responsible for the bombings that killed three people and wounded more than 170. US President Barrack Obama said it was not known if the attack was carried out by an international organisation, a domestic group or a "malevolent individual". There has been no claim of responsibility.
I am sure that more details will emerge soon, but at this stage the world seems to be in a state of shock.
Time had dulled the shock of 9/11, but Boston has made us all realise that we are still vulnerable to these attacks, no matter who is behind them.