Around one in 20 children under the age of 12 has a stammer.
That's when people get stuck on their words and struggle to speak.
Newsround's Hayley, who used to have a stammer when she was little, looked into the issue.
"From the age of about four, I had a stammer.
A stammer is when you find it really difficult to speak and struggle to get your words out. You can end up repeating a sound over and over again, or sometimes you just can't get any sound out at all.
So if you wanted to say "Hello my name is Hayley," it would sometimes sound like "H.. H.. H.. H.. hello, my na..na..name is Hayley.'
It made life pretty tricky for me sometimes but I had loads of lessons to control it, and when I did finally beat the stammer, it gave me much more confidence to go out in public and speak.
Good and bad
At the moment, about one in 20 people under 12 have a stammer, so that's quite a lot.
One of these is Kerys. She's had a stammer for quite a few years but is hoping to overcome it. Kerys came to meet me for the day and we talked about what it's like to have a speech problem.
Kerys told me there are good things and bad things about having a stammer.
"The good points are you get sympathy off your friends and it sort of makes you and your friends better friends," she told me.
"The bad sides of it are if people don't know about it, they sort of make fun of you."
There's much more being done to help children who stammer now compared to when I was a kid, and it's definitely nothing to feel embarrassed about.
Gareth Gates talks about stammering
Kerys and I went to meet singer and musical star Gareth Gates, who's one of the most famous celebrities with a stammer. We went to see him behind-the-scenes at his worldwide tour of Les Miserables, to find out how he copes with his stammer.
"It was always hard for me when I was younger," Gareth told us. "However I haven't ever let it hold me back."
And he had some good advice for Kerys too.
"I think the only advice I could give is that you are who you are, and the quicker you accept that the easier it becomes," he said.
Hayley with Kerys
I think that Gareth is definitely proof that a stammer doesn't have to get in the way of your dreams.
Kerys said she was really nervous about meeting Gareth Gates, but she was impressed by what he had to say and will definitely use his advice.
Doctors still aren't exactly sure what causes stammering. But it's thought that if you get help early on then it can be controlled and stammering can become just a distant memory."