Where are they?: And why haven't we found them yet?

Does Alien life exist out there? The purpose of this book is not to take one side or the other in that argument. It is to explore the present state of knowledge and to say where humanity now stands on the question of whether or not we are alone in the universe. Because there isn't the slightest doubt: that is a question that has occupied humans since they became human, and it's a question that shows no sign of going away.And if there are intelligent beings elsewhere in the universe - where are they?