HARD Science Fiction, or Hard SF, is a sub-genre of Science Fiction characterized by an interest in scientific detail or accuracy, being the opposite of soft Science Fiction. It is one of several Science Fiction themes.

There is a great deal of disagreement among readers and writers over what exactly constitutes an interest in scientific detail. Many Hard SF stories focus on the natural sciences and technological developments, but many others leave technology in the background. Others contend that if the technology is left in the background it is an example of soft Science Fiction. Another distinction within the genre revolves around portrayals of the human condition. Some authors seek to reflect technical accuracy within an advanced, nearly utopian society in which mankind has attained victory over most human ills; others seek to portray the impact of technology on the human race with human defects still firmly in place and sometimes even magnified.

A few good examples of Hard SF titles are: Arthur C. Clarke‘s Rendezvous with Rama, Kim Stanley Robinson‘s Mars Trilogy, or Seveneves by Neal Stephenson.