The purpose of a second edition of Where Are They? is to enlarge upon and update issues that were debated in 1978 at a two-day Symposium of the same name. As might be expected, comparison of the present book with the first edition shows that relatively little has changed in the field of interstellar travel and colonization – there are not many interstellar travelers among us. By way of contrast, because the world is full of biologists and astronomers, there have been many new experiments and insights in these fields. We are especially pleased that three distinguished biologists – Drs Diamond, Joyce and Mayr – have contributed new chapters to the present volume. Typically, biologists appear to be less sanguine about the likelihood of abundant intelligent life in the universe than are engineers and physicists.

At the time of publication of the first edition, the only things that humans knew of with certainty that orbited stars other than the Sun were other stars. Then, in 1983, NASA's IRAS satellite discovered that many nearby stars similar to our Sun emit infrared (heat) radiation well in excess of that expected from their visible surfaces. In a chapter in the first edition entitled ‘Searches for electromagnetic signals from extraterrestrial beings’ I discussed the possibility that IRAS might discover a so-called ‘Dyson Sphere’.