48 guest stars you might have forgotten were on 'Grey's Anatomy'

• "Grey's Anatomy" is in its 15th season.
• A number of stars have made guest appearances on the series. 
• Celebrities including Demi Lovato, Mandy Moore, and Elisabeth Moss have all appeared.  

"Grey's Anatomy" has aired more than 300 episodes over the course of 15 seasons, which means there are a ton of opportunities for guest stars.

Over the seasons, stars like Demi Lovato, Mandy Moore, and Elisabeth Moss have made guest appearances on the show. And some actors, like "Stranger Things" star Millie Bobby Brown, have hit it big after appearing on the ABC medical drama. 

Here are 48 stars you might not remember have been on the show. 

"Grey's Anatomy" returns January 17 at 8 p.m. EST on ABC.

SEE ALSO: WHERE ARE THEY NOW: 12 actors who have been killed off of 'Grey's Anatomy'

FOLLOW US: INSIDER entertainment is on Facebook

"The Handmaid's Tale" star Elisabeth Moss appeared on an early episode as the daughter of a patient.

Moss played a character named Nina Rogerson on episode 19 of season three. Nina is very close to her mother who is in the hospital and needs a risky surgery. She's devastated after her mother dies.

Before "Stranger Things," Millie Bobby Brown had a guest spot on an episode on season 11.

She played Ruby, a young girl who saves her mother's life. After her mom gets injured, she calls the hospital, and the doctors walk her through a life-saving procedure. She then spends time applying CPR until a helicopter arrives and rescues them both.

Mandy Moore already had an established music and acting career when she appeared on four episodes of the show over two seasons.

Moore played Mary Portman. She's first introduced on the two-part season six finale. She's in the hospital for a colostomy bag reversal surgery when the hospital is shut down because of an active shooter. She and her husband decide to travel and return to the hospital for the surgery six months later. She ends up in a coma after her surgery and her husband takes her off of life support.