Time Lords do not age the same way most life-forms do and are capable of regenerating upon 'death'. This regeneration process completely changes a Time Lord's physical appearance and voice and often alters certain aspects of their personality. A Time Lord can supposedly regenerate twelve times throughout his life. The Doctor is now on his twelfth life. Below is a complete list of the Doctor's regenerations.
The First Doctor
Portrayed by William Hartnell. The Doctor's original incarnation was already an old man when he first appeared at the beginning of the series in 1963. This incarnation met his end in the classic story The Tenth Planet, during his first ever encounter with the Cybermen on their homeworld of Mondas. He was physically drained during this encounter and his body succumbed to its age, initiating his first regeneration.
The Second Doctor
Portrayed by Patrick Troughton. The second Doctor, prior to his next regeneration, had taken up residence on Earth and lived in considerable luxury for a time, assisting people in need as they brought their troubles to him. The Council of Time Lords eventually tracked him down, however, and sent their servants (who resembled scarecrows) to punish him. The Doctor was shot and he was forced to regenerate, but was given a choice of different forms for his new body. He rejected them all and so the Time Lords chose a body for him.
The Third Doctor
Portrayed by Jon Pertwee. This version of the Doctor spent most of his time on Earth in the employ of UNIT, a UN-sanctioned military organisation dedicated to preserving world peace and monitoring the skies for alien activity. During his third life, the Doctor would also frequently come into conflict with rival Time Lord, The Master. He would meet his fate during Planet Of The Spiders, where he would expose himself to dangerous levels of radiation as he attempted to destroy the Great One, leader of the Spiders. Following this, the Doctor returned to the TARDIS and wandered the Time Vortex for a decade until he finally regenerated into his fourth body.
The Fourth Doctor
Portrayed by Tom Baker, the fourth incarnation of the Doctor had the longest television run of all of the character's portrayals and is largely considered to be the most popular. In the story Logopolis, the Doctor was forced into conflict against the Master, who was prepared to conquer the universe by threatening it with entropy. The Doctor thwarted the Master's scheme, but the entropy caused his body to expire, triggering a new regeneration.
The Fifth Doctor
Portrayed by Peter Davidson. The Doctor's fifth incarnation perished in The Caves of Androzani, after he and his companion Peri contracted Spectrox toxaemia when exposed to the mineral spectrox in its purest form. The Doctor possessed a cure for the deadly disease but gave it to Peri, allowing himself to succumb to the condition and regenerate into his sixth body.
The Sixth Doctor
Portrayed by Colin Baker. The Doctor spent most of his sixth life on his homeworld of Gallifrey after being put on trial by the High Council of Time Lords. He survived his trial, but would meet his fate in Time And The Rani when villainous Time Lady, the Rani, attacked the TARDIS. The Doctor was grievously wounded in the attack and was forced to regenerate.
The Seventh Doctor
Portrayed by Sylvester McCoy. The Doctor Who series was cancelled before the seventh Doctor completed his run, but he would appear again early in the TV movie Doctor Who: The Movie. The TARDIS was forced to land in San Francisco, 1999 following the Master's escape from confinement. When the Doctor exited the TARDIS to survey his surroundings, he was immediately gunned down when caught in the crossfire between two street gangs. His body was picked up by paramedics and taken to a hospital morgue, where he regenerated inside a cadaver drawer.
The Eighth Doctor
The Doctor's eighth incarnation - portrayed by Paul McGann - only ever appeared during Doctor Who: The Movie. Following the movie, the Doctor's adventures were only ever published in novels, comic books and audio shows. In 2013, during the run-up to Doctor Who's 50th anniversary special, BBC produced a micro-webisode that revealed the fate of the Eighth Doctor. The Time Lords were at war with the Doctor's oldest enemies, the Daleks, and the Doctor feared that their war would only end with the complete annihilation of all of creation. Regardless, he actively avoided joining the fight for several years. However, the fighting intensified with the passage of time, allowing the Doctor few opportunities to rest in his efforts to aid the parts of the universe that had been badly affected by the war. Unable to spare much time to care for himself, his appearance degraded from tidy to scruffy. His TARDIS also showed similar degradation through heavy damage, marred with scorch marks from laser fire and a weathered exterior from the battlefields that it had escaped. Eventually he was brought to a crashing spaceship above Karn, where he met and tried to save Cass, a pilot who would rather die than accept help from a Time Lord.
The Doctor died in the crash, but was revived temporarily by the Sisterhood of Karn, and was told he had under four minutes to live. They offered him control of his regeneration so that he could become the person he needed to be to end the Time War. Initially he refused their offer, but upon seeing the dead body of Cass, and being asked by the head sister how many others like her he would allow to die in the conflict, he gave in and, deciding there was no longer a need for the Doctor in the universe, he accepted the sisterhood's help, asking them to make him a warrior. The head sister then handed him a formula she had specially prepared for that purpose. Before ingesting the chalice of regenerative serum, he commanded the sisters to get out of the room, horribly torn apart by what he was about to do. Before ending his life, he paid tribute to some of his past companions.
His last words were despairingly uttered; "Physician, heal thyself." The Eighth Doctor then downed the chalice and began a painful regeneration, gasping out his last breaths in convulsions that made him keel over to the ground. When the regeneration concluded, his successor promptly rejected the name of the Doctor, declaring, "Doctor no more".
The War Doctor
The War Doctor - or "The Renegade" as he called himself - was technically the Doctor's ninth incarnation, but his future selves would attempt to bury the memory of his existence, ashamed of the things that this particular incarnation had done during the Time War. The War Doctor was portrayed by John Hurt and only appeared during the 50th anniversary special, "The Day of the Doctor".
The War Doctor died of old age, just as his first incarnation had, but not before he enjoyed a spectacular adventure with his tenth and eleventh incarnations as they prevented the Zygons from conquering Earth in the 21st century and even managed to change the outcome of the Time War by suspending Gallifrey in a Time Lord stasis cube.
The Ninth Doctor
In 2005, Doctor Who returned to television and introduced the Doctor's ninth incarnation, portrayed by Christopher Eccleston. Eccleston only played the Doctor for one season, however, and regenerated in The Parting Of Ways after absorbing the energy of the Time Vortex from his companion Rose Tyler, who had used that same power to destroy a Dalek fleet.
The Tenth Doctor
Portrayed by David Tennant. This incarnation of the Doctor is quite possibly the craziest and most outwardly immature, but inside he still feels deep pain from the millions of deaths he caused during the Time War. The tenth Doctor was plagued by foreshadowing of his death since the beginning of the new series' fourth season and he finally met his fate in the two-part Christmas/New Year special, The End of Time. After preventing the return of the Time Lords and thwarting their plan to annihilate all of time, the Doctor found himself saving the life of Wilfred Mott - grandfather of his former companion Donna Noble - who had been trapped inside a radiation control booth that was about to be flooded with radiation. The Doctor could only save Wilf by entering the adjacent booth and subjecting himself to the radiation instead. After doing this, the Doctor spent his final hours catching up with his former companions, then returned to the TARDIS where he allowed the regeneration process to begin.
The Eleventh Doctor
The Eleventh Doctor was portrayed by Matt Smith from 2010 - 2013. He met his end during the Christmas special "The Time of the Doctor", protecting the small town of Christmas on the planet Trenzalore from multiple alien attacks for over 900 years. The reason so many of his enemies were attacking this small planet was because of the presence of a crack in the fabric of time. On the other side of this crack, the Time Lords - having previously been spared from total annihilation at the end of the Time War - were sending a message calling for the Doctor to bring them back. The Daleks, Cybermen, Sontarans and other races the Doctor had previously fought wanted to seal the crack completely and stop the Time Lords from ever returning, though the Doctor was also unwilling to bring his people back for fear the Time War would begin anew and result in the destruction of the entire universe.
After 900 years of protecting the town of Christmas and using his remaining regenerative energy to sustain his current body, the Doctor was reduced to a feeble and withered old man. By this point, he had run out of tricks to confound his enemies, most of whom by then had retreated and given up the siege, all except for the Daleks. The Doctor was ready to accept his fate, but his companion Clara Oswald - who had managed to return to Trenzalore thanks to Tasha Lem taking control of the TARDIS - was able to communicate with the Time Lords through the crack in time and begged them to save the Doctor. The Time Lords evidently responded and granted the Doctor a new cycle of regenerations. With the regeneration energy he had been blessed with, the Doctor destroyed the remaining Dalek forces. The Time Lords then sealed the crack from their side and the Doctor and Clara returned to the TARDIS. The Doctor then succumbed to his age and regenerated.
The Twelfth Doctor
The latest incarnation of the Doctor is portrayed by Peter Capaldi.