Speciation has always been one of the biggest challenges in evolutionary biology, and the nature of species has been one of the most contentious. Perhaps this is why so few scientists have attempted to write major texts on the topic. Since Mayr's (1963) landmark volume Animal Species and Evolution, there have only been three significant books (Grant 1971; White 1978; King 1993), none of them attempting a full coverage of the topic and all now dated. This stands in stark contrast to the enormous and growing research effort directed at the origin of species. No wonder, then, that Coyne and Orr's Speciation has been eagerly awaited. Of course, this is not strictly a single-author book but it certainly does provide a unified and authoritative voice. If its messages are heeded, it will bring much-needed order and rigor to the current burst of activity. Speciation has a standard approach in each section. Coyne and Orr outline a problem, analyze the available theory, present the evidence with a concentration on the best studies rather than any attempt to be comprehensive, and then sum up the state of the field. There is a risk, common to major synthetic volumes like this, that many authors will use the examples cited here and not make the effort to consider those that are not. Coyne and Orr recognize that much valuable work had to be omitted. Systematic and incisive analysis is what makes the book so powerful. Take, for example, the question of the frequency of occurrence of polyploid speciation in plants and animals. It is often said that the rarity of animal polyploids is a result of their genetic sex-determination mechanisms, but this argument is flawed since it is only the relatively rare X-autosome balance form of sex determination that is an obstacle to polyploidy. Speciation by polyploidy actually seems to be common in dioecious plants. Alternative arguments have been based on differences in development, rates of hybridization and rates of selling, but none of these is sufficient to explain observed patterns. Coyne and Orr discuss two additional consequences of heteromorphic sex chromosomes that may be important. Dosage compensation is a necessary consequence, at least where the sex chromosomes are not very small, and it is disrupted by polyploidy. Rapid evolution of genic incompatibilities follows from hemizygosity of sex-