All this week on Newsround we're looking at something that worries you, as part of our Why Worry? series.
Lizo's been finding out about terrorism, what exactly it is, and how likely it is to affect your lives.
I was on my way into Newsround by Tube one Thursday morning nearly three years ago, when terrorists set off bombs on three other Underground trains and a bus.
It brought home to me how terrorism can affect all our lives.
Later that day, I reported on Newsround what the police and security services were doing to try to make sure there weren't further attacks, and how they were working to make the country safer.
Since then I've spent a lot of time finding out what's been done to try to stop terrorism here.
Terrorism isn't new, it's been around for hundreds of years.
It's the use of violence, like bombs, to try to frighten people into behaving in a certain way.
I know lots of you become more worried after a big attack, they get lots of attention and there are pictures of what's happened on TV and in the newspapers.
But it's important to remember that one of the reasons they get so much attention is because they're so unusual.
One of the people who's job it is to make sure we feel safe is Tony McNulty, who works for the Government.
He told me that when it comes to terrorism you "shouldn't worry about it to the point of making yourselves very worried or ill about it".
And that there are "lots of people" in all sorts of areas who are doing their best to make sure that terrorism doesn't interfere with our lives.
So the secret intelligence services, police and communities are all working hard to stop attacks.
We often see high security at places like airports and railway stations. But there's stuff we can do to make the country a safer place.
And if you ever spot anything that worries you, you should tell someone like a parent or teacher.
Attacks have been stopped in the past because people have reported things they find suspicious.
And things like this are an important way of making sure the UK stays safe.