The Master's regeneration
The Master regenerates The Master has regenerated once on-screen in the 2007 series episode "Utopia". After being fatally shot by the insectoid creature Chantho, the Master regenerates from the incarnation known as Professor Yana, played by Derek Jacobi, into the current incarnation, played by John Simm. Before regenerating, the Master expresses desire to become "young and strong" like the Tenth Doctor. The effect used for the Master's regeneration, though similar to the Ninth, Tenth and Eleventh Doctor's, is notably brighter and more colourful, using an array of psychedelic colours.
Previously, the Master has been shown to possess non-Gallifreyan bodies in order to extend his life. The first was a Trakenite named Tremas in The Keeper of Traken and the second was a human named Bruce in the TV movie. In The Deadly Assassin it is stated that the Master had in fact used up all his regenerations, hence his decrepit appearance in that serial. He is in fact attempting to use the artifacts of Rassilon to obtain a new cycle, but the process would destroy Gallifrey, so the Doctor intervenes. In The Five Doctors the High Council of Gallifrey offers the Master, who is now possessing the body of a Trakenite, a new regeneration cycle in exchange for his help. Although there's no indication he actually received this new cycle, in "Utopia" he regenerates naturally and in '"The Sound of Drums" he indicates that he had been resurrected by the Time Lords to fight in the Time War, suggesting his life had indeed been extended.
In "Last of the Time Lords", the Master demonstrates that regeneration is not an automatic process (or, it is, but can be prevented by will), refusing to regenerate after being shot by Lucy Saxon, despite the Doctor's pleas. The Master returned in The End of Time, resurrected with the same body he died in. He later makes remarks bemoaning this fact, suggesting this was not intended.
Previously, the Master has been shown to possess non-Gallifreyan bodies in order to extend his life. The first was a Trakenite named Tremas in The Keeper of Traken and the second was a human named Bruce in the TV movie. In The Deadly Assassin it is stated that the Master had in fact used up all his regenerations, hence his decrepit appearance in that serial. He is in fact attempting to use the artifacts of Rassilon to obtain a new cycle, but the process would destroy Gallifrey, so the Doctor intervenes. In The Five Doctors the High Council of Gallifrey offers the Master, who is now possessing the body of a Trakenite, a new regeneration cycle in exchange for his help. Although there's no indication he actually received this new cycle, in "Utopia" he regenerates naturally and in '"The Sound of Drums" he indicates that he had been resurrected by the Time Lords to fight in the Time War, suggesting his life had indeed been extended.
In "Last of the Time Lords", the Master demonstrates that regeneration is not an automatic process (or, it is, but can be prevented by will), refusing to regenerate after being shot by Lucy Saxon, despite the Doctor's pleas. The Master returned in The End of Time, resurrected with the same body he died in. He later makes remarks bemoaning this fact, suggesting this was not intended.