Susan

Susan (last name unknown) is the of Penny and  of Wyatt. She also has two other, one named Randall and a second girl who we have never met. She lives with her outside of, Nebraska on their small  where they raised their three children.

She was the only mother of any members of the gang who hadn't never been seen on the show for quite long until she finally appeared in the first episode of Season 10/208th episode "The Conjugal Conjecture", except for Stuart Bloom's mother who up to that point is only mentioned and Emily Sweeney's mother and Leslie Winkle's mother who both haven't been seen or mentioned.

Overview
In "The Flaming Spittoon Acquisition", Sheldon implies that she's fat (as well as in "The Pulled Groin Extrapolation" where he states he has seen of her and sarcastically encourages Penny to keep eating) and Penny reveals that her mother smoked  when she was  with her.

In "The Transporter Malfunction", Leonard and Sheldon told Penny they didn't want to open the Star Trek she had just bought them because once you open the box, it loses its value. Penny relied, "Yeah, yeah, my mom gave me the same about my . I gotta tell you, it was a lot more fun when I took it out and played with it."

In "The Status Quo Combustion" Penny calls her mother to tell her about her to Leonard. She denies that she is and agrees that it is a first for their.

In "The First Pitch Insufficiency", Amy wants to know when Penny and Leonard are getting. They haven't set a date and Penny wants to wait long enough to prove to her mother that she's not pregnant.

In "The Bachelor Party Corrosion", Penny tells her that she  with Leonard. Her mother was out with Penny's brother Randall who was celebrating being for one  and had received his 10th chip on account of being sober for a month. Her mother was going to make her son a little to commemorate the occasion.

In "The Conjugal Conjecture" Susan visits California and worried about her son that is on parole and what Leonard's family thought about them.

Trivia

 * Kaley Cuoco originally had stated that she would like to have play Penny's motherLisa_Kudrow_2.jpg.
 * When Penny talks to her mother about being mother she tells her mother that she is not pregnant which was a first for their family.
 * She had already met Leonard off-screen before her first appearance. This was hinted in "The Bachelor Party Corrosion" when Penny, Bernadette and Amy are discussing Penny waiting to tell her she got  to Leonard, Amy says "Well, it's kind of sad that they don't know. I mean, they love Leonard." referring to Wyatt and Susan.
 * According to a July 2016 article on the TBBT page, actress Katey Sagal has been cast as Penny's mother, Susan, and will be seen in the Season 10 premiere. This will be a  of sorts for Sagal and Cuoco, who played mother and daughter on the sitcom . Kaley has stated that Katey is her "TV mom".