SETI is the acronym for Search for Extra-Terrestrial Intelligence, organized efforts by humans to detect intelligent aliens. A number of efforts with "SETI" in the project name have been organized, including projects funded by the United States Government. The generic approach of SETI projects is to survey the sky to detect the existence of transmissions from a civilization on a distant planet. (This approach is not science fiction.)

There are great challenges in searching across the sky to detect a first transmission that can be characterised as intelligent, since its direction, spectrum and method of communication are all unknown beforehand. SETI projects necessarily make assumptions to narrow the search, and thus no exhaustive search has so far been conducted.

SETI@home is a highly successful distributed computing project that was launched by U.C. Berkeley in May 1999, and is heavily sponsored by The Planetary Society. Any individual can become involved with SETI research by downloading and running the SETI@home software package, which then runs signal analysis on a "work unit" of data recorded from the central 2.5 MHz wide band of the SERENDIP IV instrument. The results are then automatically reported back to UC Berkeley. Over 5 million computer users in more than 200 countries have signed up for SETI@home and have collectively contributed over 19 billion hours of computer processing time.