21 celebrities you forgot guest starred on 'Criminal Minds'

• "Criminal Minds" is about to debut its 15th season.
• There are lots of characters that come and go in each episode, and lots of those small roles have been played by familiar faces.
• From Jane Lynch to Elle Fanning, these are some celebrities who have appeared on the show.
• This post contains some spoilers for the show's past seasons. 

"Criminal Minds" is one of the longest running shows on television, according to Variety, and it's preparing to debut its 15th and final season sometime this year. Over the course of the program's 14 seasons, there have been countless victims and serial killers that it's almost impossible to keep track of all the transgressions. But as with any hit show, certain characters stand out more than others, especially when they're played by various celebrities.

Read on for a refresher on all the celebrity cameos in "Criminal Minds." 

SEE ALSO: 'Grey's Anatomy' guest stars over 15 years

FOLLOW US: INSIDER is on Facebook

Aaron Paul dabbled in crime in season one before his breakout role on "Breaking Bad."

One of the earliest celebrity appearances comes from "Breaking Bad" star Aaron Paul in season one of "Criminal Minds," three years before he moved on to (fictional) meth-making. Paul's role was small but memorable as Michael Zizzo, a punk-goth troublemaker who was, at one point, the prime suspect in the Behavioral Analysis Unit (BAU)'s investigation of killings and kidnappings by a group of Satanic worshippers.

Jane Lynch had a recurring role as Dr. Spencer Reid's mentally ill mother.

While she was simultaneously playing the ruthless Sue Sylvester on "Glee," Jane Lynch was portraying the schizophrenic and caring mother to Dr. Spencer Reid, Diana Reid. Lynch appeared in eight episodes over the course of eleven years of the show from 2006 to 2017 and was even the subject of a multi-episode arc that involved her character being kidnapped in season 12.

Cameron Monaghan started in the business early when he guest starred at the age of 13.

You may know him as Ian Gallagher on Showtime's “Shameless,” but Cameron Monaghan has actually been acting since he was a kid, and one of his earliest roles was in season two of "Criminal Minds." A 13-year-old Monaghan played Jeffrey Charles, the son of a small-town school counselor James Charles. While James is initially a prime suspect in a string of killings, it's actually Jeffrey that's revealed to be the culprit.