Addison Adrianne Forbes Montgomery, M.D., F.A.C.S., F.A.C.O.G., previously known as Shepherd, is a fictional character who first appeared as a supporting main character on the ABC television series Grey's Anatomy, and as the protagonist of its spin-off Private Practice played by Kate Walsh. Addison is a world-class neonatal surgeon with board certifications in both obstetrics and gynaecology and maternal–fetal medicine. In addition, she has completed a medical genetics fellowship.

Addison initially joined Grey's Anatomy in the Season 1 finale as the estranged wife of Derek Shepherd (Patrick Dempsey). Although she was originally planned as a recurring guest character, creator Shonda Rhimes grew fond of Walsh's portrayal, which led to Addison becoming a series regular by season 2 episode 7, "Something to Talk About". Addison's character played a significant role throughout seasons 2 and 3, with her final appearance as a regular in the season 3 finale, "Didn't We Almost Have It All?". Despite her departure as a series regular, Addison continued to make guest appearances over the next 5 seasons, including a notable two-part guest stint in Season 5.

After the love triangle involving Addison, Derek, and Meredith Grey (Ellen Pompeo) was resolved, Rhimes created the first spin-off of Grey's Anatomy centered around Addison, titled Private Practice. The new series saw Addison move to Los Angeles to start fresh, working at the Oceanside Wellness Group, a fictional private practice in Santa Monica, California. Private Practice ran for six seasons, from 2007 to 2013, airing concurrently with Grey's Anatomy's seasons 4 through 9.

Walsh decided to retire the character after 8 years of portraying Addison, making her final series regular appearance in the Grey's Anatomy universe during the Private Practice series finale, "In Which We Say Goodbye", on January 22, 2013. However, nearly 9 years later, Walsh returned to the role of Addison in a recurring capacity for the Seasons 18 and 19 of the original series.

Most of Addison's past is revealed in a non-linear fashion, primarily throughout the second season of Grey's Anatomy and the second and third seasons of Private Practice. Addison comes from a wealthy background and has a $25 million trust fund. Her father, nicknamed "The Captain", is a doctor who teaches medicine at a university, where Addison would watch him as a child. However, her father's time with Addison was often a pretense for his numerous affairs. Her mother, Beatrice "Bizzy" Forbes Montgomery, is also a prominent figure, and her brother, Archer, is a renowned neurologist and a successful author.

Addison met her future husband, Derek Shepherd, in medical school, and their love story began during their time at Columbia University College of Physicians and Surgeons in New York City. Addison is also a graduate of Yale University. During her time in New York, she formed close friendships with Sam and Naomi Bennett, who would later become her colleagues at the Oceanside Wellness Group in Private Practice.

Despite their early connection, Addison and Derek's marriage became strained as they started their medical practices in New York City. This professional life contributed to the growing distance between them. The couple's troubles were further compounded by Derek's family disapproving of Addison, particularly his mother, who gave her deceased husband's ring to Derek's second wife, Meredith Grey. Additionally, Addison's brother, Archer, disliked Derek, which created further tension.

The pivotal moment in their relationship came when Addison had an affair with Mark Sloan, Derek's best friend, further damaging their marriage. Addison briefly tried to build a relationship with Mark, but it quickly fell apart when she discovered she was pregnant and learned that Mark had cheated on her. She decided to have an abortion, still being in love with Derek, and subsequently moved to Seattle in an attempt to repair her relationship with him, which ultimately failed.