In this, our second edition of Reproduction in Mammals, we are responding to numerous requests for a more recent and rather more detailed treatment of the subject. The first edition was accorded an excellent reception, but the Books 1 to 5 were written twelve years ago and inevitably there have been advances on many fronts since then. As before, the manner of presentation is intended to make the subject matter interesting to read and readily comprehensible to undergraduates in the biological sciences, and yet have sufficient depth to provide a valued source of information to graduates engaged in both teaching and research. Our authors have been selected from among the best known in their respective fields. Book 4 pays particular attention to genetic, environmental, behavioural and immunological mechanisms that can contribute to an animal's overall reproductive fitness, through which natural selection must ultimately operate.