When a Time Lord dies, their body regenerates.  This can involve a change of gender, skin colour or even accent!  However, a Time Lord can only regenerate a total of 12 times - unless, perhaps, you're the Doctor!

The First Doctor
Episode: The Tenth Planet - Part 4
Date: 29th October 1966
Cause: Old Age

The Second Doctor
Episode: The War Games - Part 10
Date: 21st June 1969
Cause: The Time Lords

The Third Doctor
Episode: Planet of the Spiders - Part 6
Date: 8th June 1974
Cause: Radiation Poisoning

The Fourth Doctor
Episode: Logopolis - Part 4
Date: 21st March 1981
Cause: Fell From a Radio Telescope

The Fifth Doctor
Episode: The Caves of Androzani - Part 4
Date: 16th March 1984
Cause: Contracted Spectrox Toxaemia

The Sixth Doctor
Episode: Time and the Rani - Part 1
Date: 7 September 1987
Cause: A TARDIS Crash

The Seventh Doctor
Episode: The TV Movie
Date: 27th May 1996
Cause: Shot Dead

The Eighth Doctor
Episode: The Night of the Doctor
Date: 14 November 2013
Cause: Chose To Be A Warrior

The War Doctor
Episode: The Day of the Doctor
Date: 23 November 2013
Cause: Old Age

The Ninth Doctor
Episode: The Parting of the Ways
Date: 18 June 2005
Cause: Absorbing The Time Vortex

The Tenth Doctor
Episode: The End of Time - Part 2
Date: 1 January 2010
Cause: Radiation Poisoning

The Eleventh Doctor
Episode: The Time of the Doctor
Date: 25 December 2013
Cause: Old Age

The Twelfth Doctor
Episode: Twice Upon a Time
Date: 25 December 2017
Cause: Mondasian Cybermen