<P> Derek's background was generally a mystery for the first season and source of speculation amongst his colleagues due to his sudden departure from an established and highly respected practice in New York . In the season one finale, his past eventually catches up with him when his estranged wife Addison moves to Seattle and is offered a position by Dr. Webber . Shortly thereafter, his childhood best friend Mark joins Seattle Grace as the new head of plastic surgery . Derek and Addison attempt to repair their marriage but attempts were futile . Since their divorce they have remained on amicable terms, with Addison even admonishing Meredith for breaking up with Derek in season three . In the Private Practice episode "Ex-Life" Derek finally tells Addison that his mother never liked her in the first place . He admits to Meredith that Addison cheating on him with Mark was partly his fault as an absentee husband . Addison eventually leaves Seattle for a private practice in Los Angeles, spawning the spin - off Private Practice . In the season eight episode "If / Then", Meredith dreams of an alternate universe where her mother never had Alzheimer's; Derek and Addison are still married but their strained relationship and Shepherd's disillusionment causes his career to stagnate, earning him the nicknames "Bad Shepherd" and "McDreary". </P> <P> When Derek is offered the Chief of Surgery position for the second time, he persuades the board to keep Dr. Webber on the staff . During the merger of Seattle Grace with Mercy West their relationship sours when Derek disagreed with Richard's handling of the merger and Richard begins to display uncharacteristic behavior, not unnoticed by his fellow surgeons . Derek learns from Meredith that Richard has since resumed drinking and feels forced to have him removed as Chief of Surgery . With mixed feelings, Derek offers him an ultimatum: go into rehab and possibly pick up where he left off after, or quit completely . </P> <P> In seasons three and four, Meredith and Derek's relationship becomes rocky and they each take time to date other people . Derek's plans to propose were ruined by a series of unfortunate events in season five . In the season finale, they decide to give their planned wedding to Alex and Izzie . Due to their tight schedule, they instead informally marry and Derek writes down their "promises" on a post-it note . They legalize their marriage in season seven in order to adopt Zola, a young African orphan treated for spina bifida . They briefly separate after Meredith tampers with their Alzheimer's trial, jeopardizing her career and tarnishing Derek's reputation . Zola is taken away from Meredith after a social worker finds out she and Derek are living separately . In later seasons, Derek often griped about how his subsequent interns and residents--mainly Lexie Grey, Shane Ross and Heather Brooks--did not quite measure up to Meredith . The social worker comes back and announces they are the official parents of Zola . As Meredith nears the end of her fifth year of residency, she and Derek are torn between staying at Seattle Grace Mercy West or leaving for Boston where Derek would work at Harvard while Meredith would be at the Brigham and Women's Hospital . </P> <P> Following his rescue from the plane crash that killed Mark and Lexie, Derek learns that he may only regain 80 percent of his hand's function . He comes to terms with the fact that his career as a surgeon may be over and is grateful that he is alive . When Callie Torres (Sara Ramirez), head of orthopedic surgery, tells him a more risky surgery could give him back full function of his hand or reduce its function if it goes wrong, he agrees, accepting the possibility of never again holding a scalpel . Derek recovers well and Callie clears him to return to work, but it is still weeks later that he feels ready to operate . Derek, Callie and fellow resident Jackson Avery decide to do nerve transplant for his hand . Meredith, newly pregnant with their second child, goes behind his back and calls his sisters so they can donate a nerve to him . Lizzie (Neve Campbell), Derek's younger sister, agrees to donate a nerve and the surgery is a success . </P>

When do meredith and derek officially became a couple