<P> Dr. Addison Adrianne Forbes Montgomery (formerly Montgomery - Shepherd) is a fictional character on the ABC television series Grey's Anatomy and Private Practice played by Kate Walsh . Addison is a world - class neonatal surgeon with board certifications in both Obstetrics and Gynaecology and Maternal and Fetal Medicine . Additionally, she is a medical genetics fellow . She works at the Oceanside Wellness Group, a fictional practice located in Santa Monica, California . </P> <P> Most of Addison's past is presented in a non-linear fashion, primarily revealed in the second season of Grey's Anatomy and the second and third season of Private Practice . Addison is the daughter of wealthy parents, and has a $25 million trust fund . Addison's father is nicknamed "The Captain" and is a doctor who teaches medicine at a university where Addison would sit and watch as a child . Her father used spending time with his daughter as a pretense for his many affairs . Addison's mother is Beatrice "Bizzy" Forbes Montgomery and her brother, Archer, is a world - class neurologist and successful author . Addison met her future husband, Derek Shepherd, in medical school . Although never actually stated in the series, this would mean that she attended Columbia University College of Physicians and Surgeons in New York City . She met future colleagues Sam and Naomi Bennett there as well . Addison and Derek each began their own practice in New York City, which placed a strain on their marriage . Their marriage was troubled in the years leading to Derek's decision to leave New York as it is eventually revealed in both Grey's Anatomy and Private Practice that Derek's mother did not approve of her (choosing to give her dead husband's ring to Derek's second wife, Meredith Grey) and Addison's brother Archer disliked Derek . Derek later admitted that he was at fault as he was an absent husband . Addison's affair with Derek's best friend Mark Sloan drove the wedge even further and Derek's departure to Seattle . Addison briefly attempted to have a relationship with Mark, which resulted in her becoming pregnant . She had an abortion as she was still in love with Derek, and when Mark cheated on her, she moved to Seattle . </P> <P> The character Addison first appears in the season one finale of Grey's Anatomy, arriving at Richard's behest . She tries to reconcile with Derek, despite his relationship with Meredith Grey, but he is still angry with her, and there is a period of antagonism between them . Addison's presence, however, does reinvigorate their relationship, and ultimately leads to Derek choosing Addison over Meredith, despite Derek eventually admitting that he fell in love with Meredith . They attempt to return to their former life, taking residence in his trailer, and Addison becomes the head of a unique surgical service integrating neonatal surgery and obstetrics and gynecology . She realizes that Derek still has feelings for Meredith, and their marriage is tested further when Mark arrives in Seattle looking for Addison . Mark makes Addison realize that Derek is in love with Meredith and that he (Mark) does love her . Derek sleeps with Addison after he sees that Meredith has moved on . When Addison finds out that Derek had sex with Meredith, she gets drunk and sleeps with Mark . He asks for a relationship, but she rejects him . </P> <P> During season two, Addison punishes Alex Karev, by putting him on her service, because he almost got her arrested . However, by season three, she begins to feel an attraction towards Alex and eventually sleeps with him, only to discover that he is not interested in a relationship with her . Feeling alone, Addison decides to have a baby and visits Naomi, who is a fertility specialist . This introduces Addison's transition from Grey's Anatomy to its spin - off, as it serves as the backdoor pilot for Private Practice . Soon after, Addison decides to leave Seattle and move to Los Angeles, joining Oceanside Wellness, led by Sam and Naomi . This marks Addison's departure from Grey's Anatomy as a regular, although she makes later guest appearances . </P>

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