This book brings together most of the information available concerning two species that diverged 2-3 million years ago. The objective was to try to understand why two sibling species so similar in several characteristics can be so different in others. To this end, it was crucial to confront all data from their ecology and biogeography with their behavior and DNA polymorphism. Drosophila melanogaster and Drosophila simulans are among the two sibling species for which a large set of data is available. In this book, ecologists, physiologists, geneticists, behaviorists share their data on the two sibling species, and several scenarios of evolution are put forward to explain their similarities and divergences. This is the first collection of essays of its kind. It is not the final point of the analyses of these two species since several areas remain obscure. However, the recent publication of the complete genome of D. melanogaster opens new fields for research. This will probably help us explain why D. melanogaster and D. simulans are sibling species but false friends.