Astrobiology Magazine has finished publishing an excellent series of presentations given at at a public forum entitled "The Drake Equation Revisited," sponsored by the NASA exobiology branch, which addressed the questions of estimating the probablities for finding intelligent life elsewhere in the universe. The speakers include Frank Drake, who formulated the equation 42 years ago and is the director for the Center for the Study of Life in the Universe at the SETI Institute, paleontologist Peter Ward, co-author of the book Rare Earth, and astronomer David Grinspoon, author of the forthcoming book Lonely Planets: The Natural Philosophy of Alien Life.

In "Part 1: The Drake Equation Revisited", Dr. Drake explains the history and the content of his famous equation.
In "Part2: Our Lonely Galaxy", Dr. Ward talks about why he thinks humanity is unlikely ever to make contact with or detect communications from other intelligent species in our galaxy.
In "Part 3: Cause for Optimism", Dr. Grinspoon responds to Dr. Ward's comments, explaining why he is optimistic about the possibility of making contact with intelligent extraterrestrials.
In "Part 4: Is Intelligence a Biological Imperative?", the three participants respond to audience questions about the biological aspects of the Drake equation.
The series wraps up with "Part 5: Galactic Civilizations", in which the three participants respond to audience questions about the evolution of machine intelligence and the potential for civilizations that span the galaxy.