Whilst beliefs in extraterrestrial intelligence have been expressed for centuries it is only recently that the technological means of conducting a search have been available. The discovery of electromagnetic radiation in the nineteenth century and the invention of the radio telescope in this century appears to have moved SETI from the realm of speculation to scientific enquiry. The early modern pioneers of SETI in the late 1950's were Guiseppi Cocconi, Philip Morrison, Frank Drake and Carl Sagan, who marshalled arguments based on what we know about terrestrial life and its origins, and applied them to astronomical data on conditions elsewhere in the universe. In 1959 SETI achieved scientific respectability when the scientific journal, Nature, published a paper by Cocconi, a cosmic ray specialist, and Morrison, a physicist. This paper addressed the question of radio communication from ET sources. The problem of communicating with ET's is that first there has to be an agreement on a medium, which in the 1950's was radio, and then agreement on a frequency. If it is assumed that ETs want to communicate by radio, then the problem of which noise-free frequency out of millions has to be considered. Cocconi and Morrison proposed a frequency which was based on the rate at which hydrogen atoms emit radiation when the spin axis of the electron orbiting the nucleus flips over from being parallel to the nucleus's spin to being opposite to it. They argued that the hydrogen frequency "has a universal uniqueness, not set by anthropocentric considerations, that fits it as the outstanding choice for potential communicators who have not had the opportunity to agree on a frequency" (Cocconi and Morrison 1959, p. 844).