The Doctor is a Time Lord, and one of the peculiarities of his race is that they "regenerate" when their bodies wear out, taking on a different physical form and personality. A Time Lord's natural lifespan is twelve regenerations (thirteen incarnations).

In the classic televison series (1963 - 1989), the Doctor went through his first seven incarnations - regenerating a total of six times (this does not include the version of the Doctor played by Peter Cushing in two movies, which do not fit into canon).

The modern era began in 1996 with a movie by Fox Studios which introduced an eighth incarnation. And in 2005 a ninth Doctor returned to regular television viewing, as the BBC began production of a modernized continuation of the original television series. The tenth incarnation of the Doctor (ninth regeneration) was introduced in the 2005 Christmas special and the eleventh Doctor debuted in April 2010.