A generation ship is a hypothetical type of starship that would travel much slower than light across great distances between stars (see interstellar travel). Since such a ship might take thousands or tens of thousands of years to reach even nearby stars, the original occupants would die during the journey, leaving their descendants to continue traveling.

It is estimated that, in order to assure genetic diversity during a centuries-long trip, any generation starship would require at least 500 inhabitants (though this could also be achieved for a much smaller crew through the use of sperm banks or egg banks brought along for the journey). Additionally, the ship would have to be almost entirely self-sustaining so as to provide food, air, and water for everyone on board. It must also have extraordinarily reliable systems that would not fail even over long periods of time, or alternately that could be repaired by the ship's inhabitants if they did.