Science fiction is the genre that asks what if? It is the genre of infinite possibilities. It covers everything from time travel, to alien encounters, to the farthest reaches of the human imagination.

Subgenres:
As science fiction is a genre defined by content not pattern, the variety in the subgenres is enormous. The following are some popular subgenres as defined by theme.

Adventure
Space Opera • Militaristic and War • Exploration

High Tech
Robots, Cyborgs, Androids • Computers • It's Alive! Artificial Intelligence • Virtual Reality • Nanotechnology

The Future is Bleak
Nuclear Annihilation • Overpopulation and Plagues • Ecological Disasters • Science Runs Amok • Death from the Heavens • Is Life Worth Living?

More than Human
Bioengineering • Psionics • Immortality

Journeys Through Space and Time
Space Flight • Epic Journeys • A Means to an End • Way Back - Way, Way Back • Superiority and Persecution • Forward into the Future • To Infinity and Beyond

We're All Just Human (Mostly)
Earth's Children • Building Other Worlds

Aliens All Around Us
F irst Contact (The Search, Friends, Foes) • Aliens Among Us (Extra-Terrestrial Pals, Villains) • Encounters in Space (Fighting BEMS in Space, Alien-Human Cooperation, Close Encounters of the Alien Kind)

Social Structures
Religious • Political • Biological • Cultural

A Broken Butterfly Wing
Parallel Worlds • Alternate History

A Funny Thing Happened on the Way to Sirius

All's Fair in Love and Space
Science Fiction Spiced with Romance • Romance in a Science Fiction World

A Gumshoe by Any Other Name

Shared Worlds

The Golden Age of Science Fiction is 12
Childhood's Favorites • For Teens About Teens

To find books in these subgenres and themes, check your ultimate science fiction guide.