Many discussions of the biological world use the terms complex or complexity. But what is the real sense of these words? Conceptually the usage of complexity is slippery and is often used synonymously with diversity. In any system consisting of many components, its diversity, in the sense used in this book, is a measure of the number of different types of entity present and does not, on its own, necessarily imply any interactions between them. The components of a system could be entirely independent. However, complexity is a different beast and is admirably exemplified by biological systems. In a complex system the different components interact with, and influence the behaviour of, other components of the system. In this sense the complexity of a system is a measure of a functioning whole.