It is the world's largest student-run philanthrophic event, raising money for pediatric cancer.
The event raised more than $10 million in 2019.
Dancers are not allowed to sleep - or even sit down - during the 46 hour dance marathon.
Child cancer survivors and their families also participate along with the dancers.
"I lost one of my childhood best friends to cancer when I was in high school," said Olivia Torres of Mays Landing, New Jersey.
On Friday night, we ran into Laura Bender and her family from Lancaster. In fact, we met Bender family last year.
Unfortunately, their daughter, Kimberly, has had cancer since she was 2 years old and THON has meant the world to them.
"The emotional support that the kids, how they just love our kids and all the kids in Four Diamonds. It's the silver lining," said Bender.
After hitting the half-way mark of the event, Sarah Fina is trucking on through.
"I have no pain right now," said Fina.
"I have a great support system," added Kerry Lyons.
Huntingdon Valley's own Regina Duesler is in charge of this year's event. She says the theme this year is "Journey Together."
"It really represents not only the journey that our families go through when they're diagnosed with cancer but the journey THON has gone through since it was founded 1973," said Duesler. "Now, 40 some years later we're raising over $10 million a year with over 16,000 volunteers."
The money raised benefits pediatric cancer patients and their families at the Penn State Milton S. Hershey Medical Center.
The event has raised more than $150 million for pediatric cancer research.