Seventh Doctor's regeneration

"I've got to stop... him..."

- The Seventh Doctor's last words

The Seventh Doctor underwent his regeneration into the Eighth Doctor in Walker General Hospital on 31 December 1999. Following a non-fatal shooting, the Doctor had been taken to the hospital where subsequent exploratory surgery by Grace Holloway with a camera accidentally clogged a vein; the anaesthetic he had been given delayed the regeneration by several hours almost destroyed the process entirely. (TV: Doctor Who) This was the seventh regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Anticipation
After nearly drowning in the oceans of the Artifact, the Seventh Doctor briefly began to regenerate, but the process was averted when Mark Bannen applied artificial respiration. (PROSE: Parasite)

While trapped in his own mind during his confrontation with the Scourge, the Seventh Doctor and Benny Summerfield saw the Doctor's eighth incarnation, which the Doctor noted was more of a possibility than a reality when Benny became attracted to the image. (AUDIO: The Shadow of the Scourge)

After he suffered a one-sided heart attack at Roz Forrester's funeral, the Seventh Doctor had a vision of Death taunting him about taking the life of one of his companions, and that she would soon take his "without warning," when he was "alone and afraid". (PROSE: So Vile a Sin) The Doctor later noted that his regeneration was growing nearer. (PROSE: The Room With No Doors)

The regeneration
To be added

Alternate timelines
In an alternate timeline caused by the Doctor and Ace leaving future technology at Colditz Castle in 1944 which enabled the Nazis to win World War II, the Doctor was shot at a checkpoint in Germany shortly after landing in the alternate 1955. He subsequently regenerated, with it being believed his body had disappeared. The new Doctor tricked Elizabeth Klein into travelling back in time to Colditz, with her presence enabling the younger Doctor to avert history changing, negating this regeneration. (AUDIO: Klein’s Story)

In one possible course of events, the Doctor apparently died during his battle with Tannis. However, Tannis had declared that the Doctor's actions would rewrite time, (WC: Death Comes to Time) with most accounts indeed agreeing that these were not the circumstances of the Seventh Doctor's death. (TV: Doctor Who)

An elder Seventh Doctor who was brought to the Memory TARDIS told Ace that "Time streams are funny things. In some, I regenerate. In some, I don't," concluding that "It's all a matter of perspective." (TV: The Curse of Fenric)

Behind the scenes
Alixion would have ended with the Seventh Doctor regenerating due to psychic tension from facing the Manager, with Andrew Cartmel and Robin Mukherjee envisioning McCoy's Doctor regenerating "straightjacketed and gagged in a confinement cell". (DWM 255)