Alien invasion

An alien invasion is the process of any sentient species invading another planet, usually not one under its control.

Alien invasions of Earth
Many extraterrestrial species often tried to invade Earth from 20th to the 22nd century, mainly because Humans were then a developing race, and by sending space probes out into space they attracted the attention of other, more developed races. (DW: Spearhead from Space, The Christmas Invasion)

Some of the most persistant species who attempted to invade the Earth include:
 * The Nestenes (DW: Spearhead from Space, Terror of the Autons, Rose)
 * The Mondasian Cybermen (DW: The Invasion, The Tenth Planet, Silver Nemesis, The Wheel in Space, The Moonbase, etc)
 * The Daleks (DW: The Dalek Invasion of Earth, Dalek, The Parting of the Ways, Doomsday, etc)
 * The Ice Warriors (DW: The Seeds of Death, The Ice Warriors, NA: The Dying Days'')

Motives
There are many motives for alien invasion. Some species invade due to the destruction of their homeworld (DW: Terror of the Zygons, Rose, The Parting of the Ways, SJA: Invasion of the Bane, etc), or the rendering of their homworld uninhabitable (DW: The Tenth Planet, The Seeds of Death, etc), whilst others invade to expand their empires. (DW: The Christmas Invasion, etc)