Some of them are horrifically nasty.
Others are foul.
Then there are sexually aggressive comments.
But perhaps the most disturbing are the ones offering genuine criticism and advice.
The weird thing is, the girls uploading these videos aren't stupid.
They're part of an internet-savvy generation. They've seen the other videos. They know that uploading their own and putting themselves up for scrutiny will invite brutal criticism from internet trolls.
And yet they do it anyway.
Louise Orwin, a 26-year-old artist from London, decided to explore this disturbing phenomenon.
So Orwin created three teenage alter egos and uploaded two-minute videos of each of them to YouTube.
All three asked the same question: Am I pretty or ugly?
"They were called Becky, Baby, and Amanda," Orwin told BuzzFeed. "Becky was your standard rock chick, wearing heavy eye makeup and playing an emo soundtrack in the background. Baby was a Britney Spears wannabe. And Amanda was me, without makeyup, in a wig, wearing glasses."
In total, the three videos received 4,000 comments. Baby, "the most conventionally good looking of the characters," received the most the most attention, but it was the internet's reaction to Becky that shocked Louise. "About a month after I posted it, there was a real spike in interest," she said.
"I remember waking up one morning and receiving all these YouTube notifications calling me a 'cunt' and telling me to 'fucking die.'"
But the interesting thing was that the vast majority of the comments were posted within a three-hour window. "The first comment before the wave of abuse simply said: 'And so it starts.'"
Thousands of comments followed. Some simply contained foul language, but the more unsettling ones were from others who offered genuine feedback. "They told me my eyes are too close together, I have a wonky nose, I speak in a really weird way," Orwin explained.
"That's when I started to question why these girls are doing it. It just seems so masochistic. They're savvy to the internet, they've seen the other videos, and they know how internet trolls will respond. So why engage?"
The only way to find out, Orwin thought, was to upload a video of herself.
Not Orwin as Becky. Not Orwin as Baby. Not Orwin as Amanda. Orwin as Orwin.
"But the weird thing was, it didn't get any attention," Orwin said. "There were a few nasty comments, but people didn't seem to want to comment on the real me."
The implication, of course, is that the "pretty or ugly" trolling community is exclusively interested in underage women.
Louise puts this down to our society's obsession with teenage girls.
"Models look like teenage girls; women want to look like teenage girls; men want to fuck teenage girls.
"Then there's the teen movie industry, which is so huge. There's something about that which makes young girls think they have to be performers.
"And, of course, it's all tied up to reality TV culture, which perpetuates the idea that anyone can become a star. [For these girls], maybe YouTube is about that; this idea that if you present an idealised version of yourself on the internet, it might attract attention one day."
The problem is that these teenagers aren't attracting the right sort of attention.
"As far as I could tell, the majority of the commenters were older men," Orwin explained. "And a lot of them sent me private messages asking for nude photographs."
But the question that Orwin kept returning to was: Why? Why are so many teenage girls posting these videos when they know the sort of reaction it'll prompt? Why are they knowingly opening themselves up to internet trolls?
"These teenagers have grown up with avatars of themselves," she explained. "Because everyone has access to things like Tumblr, Facebook, and Twitter, these girls have grown up with mini-mes, which they polish and present to the world. They constantly self-edit."
"One of the things that comes up time and time again when I interview teenage girls is that they feel a real anxiety about their online personas," Orwin said.
"They see that they have a Facebook notification and instantly worry that an unflattering picture of them could have be uploaded. And then they get stressed about getting to a computer as soon as possible in order to de-tag it."
"But although they feel anxious about it, they aren't dissuaded from using it," Orwin said.
"The problem is, the internet is so private for these girls because their parents probably don't know they're posting these videos. But at the same time it's so public. And the way these girls express themselves makes them such an easy target for abuse."
Orwin's research has inspired her to put on a show, Pretty Ugly, which explores teenagers' conflicted relationship with publicness online. It will be performed at the Camden People's Theatre in London from 23 Oct. to 5 Nov. Click here for more information.