The Master was conceived as a recurring villain, "Professor Moriarty to the Doctor's Sherlock Holmes". And recurring he was: in the three seasons following his first appearance, in The Terror of the Autons (1971), the Master (played by Roger Delgado) appeared in more serials than not.
Following Delgado's sudden death in 1973, the Master disappeared from the series for several years.
On his next appearance, in The Deadly Assassin (1976), the Master appeared as an emaciated wreck (played by Peter Pratt under heavy make-up). Although Time Lords have the potential to postpone death by completely renewing their bodies, the ability can only be used twelve times. Unlike the Doctor, the Master had in the course of his career used up all twelve of his regenerations, and was nearing the end of his thirteenth, and presumably final, life. He attempted to seize control of an ancient power source (the Eye of Harmony, an artificial black hole maintained on Gallifrey) in an attempt to give himself a new cycle of regenerations. After being defeated by the Doctor, the Master disappeared from the series once more.
In 1981, the Master became a recurring villain again. In The Keeper of Traken, the Master (played by Geoffrey Beevers under different heavy make-up) briefly gained control of another ancient power source, using it to transplant himself into the body of a Trakenite named Tremas, overwriting Tremas' original mind in the process. Now played by Anthony Ainley, the Master appeared on and off for the rest of the series.
The Master also appeared in the 1996 telemovie that starred Paul McGann as the Doctor. In the telemovie, the Master's current body (played - for mere seconds in the final edit - by Gordon Tipple) was thoroughly executed by the Daleks, but he managed to survive by some unexplained means and steal a new one (played by Eric Roberts). The Master once again attempted to access the Eye of Harmony to grant himself a new cycle of regenerations, this time via the Doctor's TARDIS power tap, but was sucked into it and apparently destroyed.