No injuries have been reported, but the damage is estimated to be over $1-million.
Claymont Fire Company Fire Chief Chief Brandon Harris says the fire at Valley Run Apartments on the 1000 block of Stevens Road started before 4 a.m..
Neighbors say they woke up to someone pounding on their door screaming "fire."
Many of them lost everything as the fire raged through buildings and burned down the roof.
"Somebody was outside screaming, 'It's a fire! Wake up, wake up. It's a fire," said Dominique Wilson, who lives with her family in one of first floor apartments.
Wilson says her family has no idea if anything they own can be saved.
"I feel lost. I never experienced anything like this before. I'm just happy me and my family made it out safe," Wilson said.
WATCH: Action Cam footage from scene of Claymont fire
Fire crews in Claymont called in backup and dozens of trucks from Pennsylvania and Delaware responded.
The chief says 54 firefighters dumped water on the flames; it took crews more than an hour to control the fire.
"The roof collapsed into the third floor, eventually through the third floor. All firefighters were outside by that time," said Harris.
Everyone who lives in the 18 apartments damaged made it out safely too, but some say they need help. The Red Cross has been called in to assist.
"Everything's gone," said resident Darlene Hull. She says she's counting her blessings.
"I thank God for everything. None of us got hurt. Everyone's fine. And that's the only thing I can look at right now, that we made it out of there," Hull said.
The state fire marshal says he's beginning to investigate a cause which could take a while considering the extent of the damage.