The object of this book is to introduce ethology to agricultural and veterinary students. Today ethology covers many approaches to the study of animal behaviour which are connected by one unifying concept: all behaviour must be considered in relation to the ecology and evolutionary history of the species under investigation. This may seem to some to put domesticated animals beyond the scope of classical ethology but, while domestication has involved some behavioural changes, we shall see that much of the behaviour of our species of farm livestock differs little from that of their putative ancestors. It is assumed that students using this book will already have studied some physiology. It is also assumed that they are, essentially, practically minded and with this factor in mind I have discussed behaviour in terms of its function, introducing the principles of ethology within functional categories of behaviour. In order to best illustrate these principles I have taken examples from a variety of species and not confmed myself to farm livestock and domestic animals, for fundamental ethological research with these species has been patchy. However at the end of each chapter I have given a list of papers pertaining to farm livestock so that the principles of ethology can be seen in a more practical context and to develop this approach further I have also added some practical problems for discussion at the end of each chapter.