This chapter highlights and defines ‘theatricalism’ and ‘theatricality’ as critical terms, useful for understanding Roman culture. It provides examples of each, suggests how useful the terms are for describing Roman art, architecture, domestic décor, ceremonies and political life. It summarises how subsequent chapters will examine the concepts informing these terms and will use these to further out understanding of crucial aspects of Rome art and society. It also introduces the concept of ‘mixed reality’ and the practice of mnemonics, ekphrasis and phantasia as key examples of how theatricalism figured in Roman artistic, mental and cultural life.