Abby Fairgate (formerly Cunningham, Ewing and Sumner) is a fictional character from the CBS prime time soap opera Knots Landing, a long-running serial about middle class life on the fictional cul-de-sac known as Seaview Circle in Los Angeles, California. She was played by Donna Mills between 1980 and 1989. Abby was created by producer David Jacobs as one of Knots Landing's earliest characters. She debuted in the first episode of the second season. Mills remained a principal actor in the series until she left in its tenth season. She returned for the two-part series finale in 1993, and made her last appearance in 1997 when she appeared in the reunion miniseries Knots Landing: Back to the Cul-de-Sac.

Prior to being cast on Knots Landing, Mills was predominantly known for playing "damsel in distress" roles, which is why the producers didn't initially consider her. Abby's storylines focused on business dealings, affairs and family troubles. Introduced as the sister of Sid Fairgate, she was initially portrayed as a friendly, warm-hearted woman with great love for her children. In due time, she evolved into the series' main antagonist, constantly causing mayhem in others' lives. According to series creator David Jacobs, the producers always intended Abby to be a J. R. Ewing-esque character. Jacobs said, "When we were casting the role of Abby, we were looking for a homey type, someone whom the other women characters would trust. It was our intention to have this nice person gain the trust of the other women and then cause trouble for them. We didn't want the other characters and the audience to know that there was a scheming person under this nice facade. We wanted her to come on as a good person and then evolve into a vixen."

The character became known for her manipulative behavior and was often labeled a "soap vixen". She was praised by television critics, with Carolyn McGuire of the Chicago Tribune saying: "Whoever shows up as the next villain, chances are his or her work won't be easy. That's because at least one resident—Abby Ewing—knows how to dish nasty with the best of them." When asked which character he wished he had created, Mike Kelley, the creator of ABC's popular prime time soap opera Revenge, said: "Abby Ewing. Donna Mills, you rocked my world." John Mapes of The Biography Channel said, "Any great soap opera needs a great villain. While viewers may identify more with the protagonist, the villains in a serial drama always spice things up, cause trouble, and make it more fun to watch."

Mills portrayed Abby from 1980 to 1989. Prior to being cast in Knots Landing, Mills was primarily known for playing the "damsel in distress" archetype in both film and television media. The actress became somewhat famous for playing these roles, often leading to unwanted typecasting. In an interview with Jerry Buck for the Toledo Blade, Mills said: "I got tired of playing the victim. It's a more active role. Abby keeps things stirred up, and I like that." According to Jacobs, Abby was not planned when the show began. He knew that he wanted a female J. R. Ewing-esque character. However, he had a different sense of the character and who would wind up in the role. With Mills' reputation of playing the victim, he initially didn't choose her for the part.

In the early planning stages of season two, CBS executive Tony Barch suggested Mills for the role of Abby Cunningham. The character was initially conceived as the sister of Sid Fairgate (Don Murray), one of the central characters on the show, who was killed off in the next season. Casting agent Barbara Miller-Gidaly introduced Mills and Jacobs. After meeting with Miller-Gidaly, Jacobs decided to have Mills test for the role, which was unusual for an established actress. She auditioned shortly after and convinced the producers that she was the right person for the part. Jacobs said, "She came back that afternoon and she read and she was great, and it meant reshaping the role a little bit, but, um, not that much. You know, she still came in, she drove up in the Volvo station wagon. She had the kids in the back. It was very middle class."

In 1989, Mills announced her intention to leave the long-running nighttime soap after nine years as Abby. According to Mills, she wanted to take a break from acting for a while, and from Abby as well. In an interview with The Cedartown Standard, Mills explained: "I'm tired of the show. It's been too long. I'm not particularly happy with the way they've been writing Abby lately. She's too soft. I'd like Abby to get back to her old self." Mills returned for the series finale and the subsequent reunion specials (Knots Landing: Back to the Cul-de-Sac and Knots Landing Reunion: Together Again). Virginia Heffernan of The New York Times praised her scenes with fellow actor William Devane, saying: "The Jean Naté-scented banter between the actors Donna Mills and William Devane is both the daffiest and the most enlightening part of the reunion special."

I always planned her, because it was technically a spin off from Dallas everybody expected there to be a JR so I wanted to make sure there wasn't a JR. So I brought in Abby and her two children and so she was to become the villain but she was a great addition to the show. My initial thought was that it would be scenes of a marriage times four, but it became a dramatic exploration of the way people related in the post sexual revolution world and how they kept their marriages together if they could, what the relationships meant to them. To Abby it was currency, sex equaled currency, that's how she got money.

The producers always intended Abby to be the "pot stirrer". Devane, who played Greg Sumner, said that the show was looking for a woman "in high heels with the throat of the guy – that's what they wanted". He added, "It was your character that influenced all these unhappy women today who have jobs and are out there working hard, trying to raise kids by themselves, going crazy." Mark Harris of Entertainment Weekly said, "Michelle Phillips is the last of prime-time's great schemers, an essential element of the show since the departure of Donna Mills, whose Abby Cunningham was Knots' resident harpy for years."