All students and researchers of behaviour - from those observing freely-behaving animals in the field to those conducting more controlled laboratory studies - face the problem of deciding what exactly to measure. Without a scientific framework on which to base them, however, such decisions are often unsystematic and inconsistent. Providing a clear and defined starting point for any behavioural study, this is the first book to make available a set of principles for how to study the organisation of behaviour and, in turn, for how to use those insights to select what to measure. The authors provide enough theory to allow the reader to understand the derivation of the principles, and draw on numerous examples to demonstrate clearly how the principles can be applied. By providing a systematic framework for selecting what behaviour to measure, the book lays the foundations for a more scientific approach for the study of behaviour.