The Migration

The migration of a homeless group to a new home. There are many motives: they may be refugees running from the light of their burning homes, they may wish to escape a wicked or unjust land and start anew. They may be migrating for profit.

This theme can be the basis of an Epic.

If done in space the heroes can become Space Cossacks in the process.

May join forces with Fighting For a Homeland. A subtrope of The Quest in fantasy settings.

Examples of The Migration include:

Anime

Space Battleship Yamato - The original reason for rebuilding the Yamato as a spaceship was something like this, but when they learned that there was technology on Iscandar that could cleanse the earth of radiation poisoning they quickly changed the mission. some of the crew try to colonize Iscandar anyway though In the 3rd series, they are in fact sent on a mission to find a new planet for people to migrate to, and fail, but end up saving Earth again.

Literature

Watership Down
Warrior Cats had this as the plot of the second series, after the original forest was destroyed to make way for a road.
Happens several times in the Tolkien legendarium, too. First some of the elves migrated to Valinor, then some of them migrated back. Then some men migrated to Numenor, then some of their descendants migrated back. Then Hobbits migrated to the Shire.. there are probably others.
The Aeneid
Several Norse sagas
The Bible: specifically, the book of Exodus
The Cold Moons by Aeron Clement (Watership Down with badgers!)
The Emigrants.
The Kushans in the last two books of Belisarius Series
In Firesong, the third book in the Wind on Fire trilogy, the Manth tribe, after the destruction of their city, travel across the continent to find a new homeland their prophet has seen.
In Katherine Kerr's Deverry series, the humans migrated to the world of Annwn after being chased out of their homeland by the "cursed Rhwmanes". This ulitmately caused the decimation of the Elven people.

Live Action TV

Battlestar Galactica
Those aliens in Star Trek: The Next Generation (those that have the True Girlfriend that Troi's husband-to-be was looking for)

Real Life

Migration is a recurring theme in human history.
- Some times disturbances in the steepelands would cause a billiard-ball like effect where one tribe would migrate to another's territory, that would migrate to someone else' and so on.

Tabletop Games

Traveller has a lot of these too. The Sword Worlds were the migration of soldiers from a previous regime on Terra who were stranded by their government's overthrow. The best sample campaign in the GURPS Traveller series, 100 Parsecs, is about a project for preserving the Sword Worlder way of life on a distant world. Aslan are always doing this as well.
- This is in some ways the second hat of the Aslan(the first is Proud Warrior Race). Aslan have a biological obsession with land and it is one of the main basis' for status. They regularly send out what they call Ihaiti fleets of colonists who settle on planets. Sometimes they do so by force and at other times they make a treaty with locals who have land to share. A classic treaty of this kind involves military service for land.
During the fall of the Star League in the backstory of BattleTech; the Star League Defense Forces left (along with their families and anyone else who wanted to go) known space to prevent their weapons and soldiers from being used against the citizens of the Inner Sphere by the warring Great Houses. Their descendants, after a bit of a cultural shift, eventually migrated back with the objective to end the still ongoing wars through force.

Video Games

Oddworld: Abe's Exoddus.
Homeworld.
The Quarian Migrant Fleet in Mass Effect 2.
In Logical Journey of the Zoombinis, Zoombini Isle has been taken over by the evil Bloats, so the Zoombinis- under the player's control- travel through a strange continent to find somewhere to settle.

Western Animation

Ice Age opens with Scrat being stomped by the animals migrating south to avoid the ice age.
An American Tail.