One cannot help being impressed by the near-perfect camouflage of a moth matching the colour and pattern of the tree on which it rests, or of the many examples in nature of animals resembling other objects in order to be hidden (Figure 1.1). The Nobel Prize winning ethologist Niko Tinbergen referred to such moths as ‘bark with wings’ (Tinbergen 1974), such was the impressiveness of their camouflage. On a basic level, camouflage can be thought of as the property of an object that renders it difficult to detect or recognise by virtue of its similarity to its environment (Stevens & Merilaita 2009a). The advantage of being concealed from predators (or sometimes from prey) is easy to understand, and camouflage has long been used as a classical example of natural selection. Perhaps for this reason, until recently, camouflage was subject to little rigorous experimentation – its function and value seemed obvious. However, like any theory, the possible advantages of camouflage, and how it works, need rigorous scientific testing. Furthermore, as we shall see below and in this book in general, the concept of concealment is much richer, more complex and interesting than scientists originally thought.