It bears repeating from previous editions that cattle are an integral part of animal agriculture in all parts of the world. They provide the human population with products such as milk, meat, clothing, fertilizer and fuel, as well as providing draft power in many countries. Demand for some products, such as fat in meat and in milk, has declined in some markets, and in some countries (e.g., in the European Union) milk production is limited by quotas. Nevertheless, the efficient production of animal protein continues to be of vital importance. An efficient production system must maximize the output of meat and milk per unit of feed input and per animal and, in cattle production, as with other livestock systems, a high level of reproductive performance is absolutely crucial to efficient production. Efficient reproduction results in optimum calving intervals, which in turn result in optimum production of milk and calves to be reared for beef or replacements. Furthermore, cows that fail to reproduce are culled, limiting the choice of animals available to produce replacements and thus limiting genetic progress. As awareness of the ethical issues surrounding animal production increases, this wastage becomes even more unacceptable.Welfare concerns are also increasing the pressure to avoid animal ill-health and, specifically, fertility problems. This should be regarded as a positive trend since, as will become evident in the book, improved health and welfare go hand in hand with better reproductive performance.