"Cities in Flight" is a science fiction series written by James Blish that explores the concept of cities that can move through space. The series is set in a future where Earth faces overpopulation and resource depletion, prompting cities to leave the planet in search of new worlds. The narrative follows the adventures of these mobile cities, focusing on their inhabitants and the challenges they face in adapting to life in space. The series delves into themes of survival, technology, and the impact of human expansion on the universe, making it a thought-provoking exploration of humanity's future.