This chapter introduces a spreadsheet so that readers can explore the effect of overlapping items on the correlation between tests of various lengths. It discusses the interplay between psychological assessment/psychometrics and psychological theory, noting that it can work two ways. Tests can be constructed to measure characteristics which have been identified through medical research etc. In addition, psychometric analyses of behaviour can show how we might usefully describe the way in which people behave, feel and act – but not why they do so. The chapter also discusses two important concepts: traits, which are rather stable aspects of personality or ability, and mood and motivational states, which are short-lived and often influenced by internal events or the environment. It describes the concept of attitude briefly. The chapter considers some rather fundamental assumptions made whenever attempting to use any scale to measure anything. It also presents an overview of this book.