Sperm competition (by males) and sperm choice (by females) have now been well documented in dozens of species. This book provides an accessible and comprehensive overview of this research. 'Promiscuity' may be a misleading term, as it suggests that females are being less choosy in mating with multiple males; the main lesson of the book is that multiple mating by females is often a way of being more discriminating, because it allows the female to compare the quality of rival sperm directly. Sexual reproduction emerges as being anything but cooperative.