The only hint of the Eighth Doctor's regeneration is when the Ninth Doctor first saw his reflection in "Rose". Details about his regeneration are unknown. Fans have come up with theories that something happened in the Time War causing him to regenerate.
- It's been strongly implied that the regeneration occurred during the Time War (presumably as a result of whatever action the Doctor took to end it by killing all the Time Lords and Daleks). However the series has yet to explictly spell out what happened to the Eighth Doctor and whether the change happened during the Time War (the Doctor's comment after seeing himself in the mirror in "Rose" could just as easily be rationalized as him reacting to a recent haircut). It should be noted that during production of the first season, Russell T Davies gave permission to Doctor Who Magazine for it to chronicle the Eighth Doctor's regeneration into the Ninth as part of its comic strip, and the storyline had nothing to do with a Time War, so Davies was open to other circumstances causing the regeneration. The reason why it didn't see print (as outlined in the introduction to the Panini compilation Doctor Who: Oblivion) is that DWM had planned to do a "Year One" story arc featuring the Ninth Doctor travelling with then-comic strip companion Destrii, and Davies mandated that the Doctor only be shown travelling with Rose, so the regeneration was nixed and the Eighth Doctor era of the comic strip simply ended with the fate of the incarnation and Destrii left unchronicled.