Our Milky Way Galaxy is only one of 10 billion galaxies in thepresently observable universe. Our Sun is just one of some 300 billionstars in our galaxy alone. Astronomers have confirmed that the Sun andthe galaxy, which make our existence possible, are not unusual orbasically different from other galaxies and stars. A few generations ago, astronomers believed that planetary systemswere extremely rare--that our solar system and our Earth with itslife-supporting environment might well be unique. Chemists andbiologists knew little if anything about the processes that led to theorigin of life. In the last fifteen years, however, a number ofimportant discoveries have strongly suggested that there is afundamental relationship between the origin and evolution of life andthe origin and evolution of the universe. Advances in astronomy and physics have given renewed support to theconcept that planets are not rare exceptions, but are a natural partof the star formation process and may number in the hundreds ofmillions in our galaxy alone. [In December 1984, the National ScienceFoundation announced that a team of Arizona astronomers had detected apossible solar system around Beta Pictoris, a star 53 light years fromEarth.] Recent biological experiments applying natural energy sourcesto molecules have produced some of the organic building blocks thatmake up the chemistry of life. Radio astronomers have discovered thatmany organic molecules exist even in the depths of interstellar space.Elements identified in these molecules include hydrogen, nitrogen,oxygen, carbon, silicon, and phosphorus. Earth has been without lifeonly a small fraction of its age, which leads many scientists to lookupon the formation of life on other suitable planets as very likely.Once begun, and given billions of years of relative stability, lifemay achieve intelligence and, in some cases, may evolve into atechnological civilization. One direct way of testing whether intelligent life exists beyond oursolar system is to search for an artificially generated radio signalcoming from interstellar space. As an example, ultrahigh frequency andmicrowave radio signals emanating from Earth are expanding into spaceat the speed of light. This radio, radar, and television "leakage" ofours currently fills a sphere nearly 100 light-years in diameter. Thesame phenomenon would serve to announce the presence of otherintelligent life. Moreover, advanced civilizations might be operatingradio beacons, possibly to attract the attention of emerging societiesand bring them into contact with a community of long-establishedintelligent societies existing throughout the galaxy. Either type of signal (leakage or beacon) would be easiest to detectat frequencies where the background radio noise is minimal. One of thequietest regions of the electromagnetic spectrum is the "microwavewindow" that lies in the frequency band between 1000 and 10,000megahertz (MHz). It is reasonable to assume that others wishing toestablish interstellar contact by radio might choose this band. The search for extraterrestrial intelligence (SETI) is not new, havingfirst been proposed by U.S. scientists in 1959. Since that time,numerous scientific and technical studies have been made on aninternational scale, and more than 30 radio searches have beenattempted, covering only a minute area of search space. What is newtoday is the available technology. Radio telescopes on Earth aresufficiently sensitive to detect signals no stronger than some leavingEarth at distances of a thousand light-years or more. The 305 meter(1000-ft) diameter radio telescope at Arecibo, Puerto Rico, coulddetect transmissions from nearby stars that are less powerful butsimilar to our own television and radars. Advances in computers anddata processing techniques now make it possible to searchautomatically through millions of incoming radio signals each secondand, if it is present, to identify a signal transmitted by anintelligent society. The NASA SETI Program is nearing the end of a 5-year research anddevelopment phase, using existing radio telescopes and advancedelectronic techniques to develop prototype SETI instrumentation. Theprogram is being jointly carried out by the Jet Propulsion Laboratory(JPL) at Pasadena, California, and the NASA Ames Research Center atMoffet Field, California. Leading radio scientists from the nationallaboratories and academic community have also joined together in theSETI Science Working Group to assist the JPL-Ames team in developingthe instrumentation and the search strategy. The proposed plan involves two complementary search modes that aredesigned to cover a range of possibilities. One mode is an all-skysurvey that will search the entire celestial sphere over a widefrequency range (1200 to 10,000 MHz plus spot bands up to 25,000 MHz)to cover the possibility that there may be a few civilizationstransmitting strong signals, possibly as interstellar beacons. Longerobserving times may be allocated to directions that include a largenumber of stars, especially the galactic plane. The radio telescopesemployed will be the 34-meter (112-ft) diameter antennas that are partof NASA's Deep Space Network. The survey will be conducted by movingthe telescope across the sky at a constant rate. It will cover atleast 10,000 times more frequency space than all previous surveyattempts, will be about 300 times more sensitive, and will take about5 years to complete. The second mode is a high-sensitivity targeted search that will lookfor weak signals originating near solar-type stars within 80light-years distance from Earth. The objective is to examine thepossibility that nearby civilizations may have radio transmitters nomore powerful than our own. Some stellar clusters and nearby galaxieswill also be observed. The frequency range covered will be 1200 to3000 MHz plus spot bands between 3000 and 10,000 MHz. To achieve veryhigh sensitivity, the targeted search will use some of the largestradio telescopes available, including the 305-meter (1000-ft) diameterantenna at Arecibo, Puerto Rico, and the Deep Space Network's 64-meter(210-ft) diameter antennas. The number of targets covered will be muchlarger than previous searches and the range of frequencies coveredwill be thousands of times greater. The targeted search is expected totake about 3 years to complete. Current astrophysical knowledge and the available technology make theSETI observing program both timely and feasible. Timeliness alsorelates to the rapidly-increasing sources of radio frequencyinterference (RFI) in the microwave band. Portions of the microwavespectrum that directly concern SETI ar subject to allocation tonumerous users worldwide, emphasizing the need to proceed with SETIwhile it remains economically possible with our current technology. Ifthe use of the microwave spectrum continues to increase at its presentrate, the greatest exploration opportunity in the history of mankindmay be placed economically and technologically beyond our reach forthe foreseeable future.