Abigail Marjorie "Abby" Lockhart, M.D. (née Wyczenski) is a fictional character from the NBC medical drama series ER, portrayed by Maura Tierney. She first appears as a guest star in the first half of the sixth season, before becoming a main character later that season, appearing until the beginning of the fifteenth season. Tierney returned to make one final guest appearance later in season fifteen. ^{[citation needed]}

The character Abby Lockhart first appears in season 6 of ER in the episode "Great Expectations", guest starring as Carol Hathaway's (Julianna Margulies) labor and delivery nurse. Three months later, she became a regular character. She first made an appearance in February 2000 as a third-year medical student beginning her ER rotation. On her first day, she meets Dr. Luka Kovač (Goran Visnjic), her first teacher, with whom she will experience both a professional and personal relationship, and who will become the most important character in her long-running story. Lockhart's first months in the ER prove challenging for her, transitioning from OB nursing, to the ER. In that time, she sees Carter stabbed, and a medical student, Lucy Knight killed by a patient with schizophrenia. She also has had to speak to superiors Kerry Weaver (Laura Innes) and Mark Greene (Anthony Edwards) about John Carter's (Noah Wyle) growing drug addiction. Although Carter is hostile at first, he and Abby become friends over the course of the next season.

Abby's medical training had gone into hiatus and she had become a full-time obstetrics nurse to provide for her then-husband Richard Lockhart's medical school education. Once they divorced, she resumed her medical training. At the start of season 7, Weaver breaks the news that her registration for her final year of medical school has been denied due to lack of payment. Abby confronts her ex-husband, who has stopped paying her tuition because had become bankrupt. She resumes her part-time OB nursing job. Soon, Weaver offers her a full-time ER nursing position, which she accepts. After several rewarding years as a nurse, Abby gets her ex-husband to sign off on a loan so she can finish medical school. The loan defaults, but John Carter pays the deficit. After she fails her first board certification exam, she passes on her second try. This leads to the earning of her MD in the Season 10 finale. Abby's first day as a doctor begins when Pratt and Chen fall victim to a road rage incident. Abby is called in as a doctor to help save her friends' lives.

In the 2000–2001 season, Abby is visited by her mother, Maggie Wyczenski (Sally Field), who has bipolar disorder. Abby's childhood is revealed to have been very difficult: she was forced to cope with her mother's erratic behavior and long absences and had to take care of her younger brother Eric (Tom Everett Scott). Eric is introduced in later episodes. He is in the US Air Force but goes AWOL and is showing signs of bipolar disorder. He refuses to accept it and clashes with both Abby and Maggie as a result. Abby eventually reconciles with her mother during the beginning of Season 13 as Maggie is taking medications for her disorder regularly, and Abby is grateful for her support when her son Joe is born prematurely.

Abby's father, Eddie, left the family when Abby was a child, due at least in part to Maggie's uncontrollable behavior. Abby reunites with her father (Fred Ward) in season 13, when he initially approaches her under the guise of being a patient named Eddie Jackson before finally revealing his true identity to her. Abby tells him to stay away from her, although she later considers inviting him to her wedding.

Abby dates Dr. Kovač, Dr. Carter and Jake, a medical student, but eventually reunites with Kovač, with whom she has a son. They later marry. She befriends many colleagues in the ER throughout the series, co-workers such as Susan Lewis, and Kerry Weaver. Her closest friend is Dr. Neela Rasgotra, who graduates medical school with her and lives with her during some of Neela's hard times. As Carter is phased out of the show and Weaver departs midway through Season 13, Abby and Luka Kovač are the senior figures on the show until their departure in 2009.

Prior to beginning medical school at County, Abby was married to Richard Lockhart; she and Richard were expecting a child at one point, but Abby chose to have an abortion, since she was afraid the child would have bipolar disorder like her mother. Abby never told her family about the pregnancy and Richard was unaware of it until after the abortion. The marriage began to break down after a short time, with Richard frustrated at Abby's abortion and depression, and Abby angry over his insensitivity, since she was working overtime at home and at work to help pay for his medical schooling. They finally split for good when Richard cheated on her and part of the divorce agreement was that he would pay for her medical schooling. Abby chose to keep her married name of Lockhart, claiming "it's the only good thing I got out of the whole mess."

Soon after taking the position in the ER, Abby begins a relationship with Dr. Luka Kovač, who is initially a mentor figure to her until she has to quit medical school. The beginning of their relationship is strained when they are attacked by a mugger on their first date, whom Luka kills in defense. She and Luka stay together for about a year, but their relationship becomes more strained after the arrival of Abby's mother and the increasing closeness between Abby and Dr. John Carter, and a heated argument leads to their break up at the beginning of the eighth season.