This chapter introduces the distinctive character of Spanish reason of state and gives a sense of its broader contribution to the study of political thought. It develops the relationship between reason of state and history, and outlines the relevance of reason of state to recent work on the early modern uses of the past, developments in early modern historical scholarship and methods of coping with so-called ‘information overload’. This chapter introduces how this book explores reason of state in a number of theoretical and practical contexts in the seventeenth-century Spanish monarchy, by focusing on individuals who used the act of organizing information – drawn from classical and recent history, their own experience and analyses of current circumstances – to rhetorically present the condition of necessity. Furthermore, the chapter highlights the paradox that although reason of state as a discourse was closely entwined with political practice, no attempt has been made to systematically trace how the discourse functioned in political action. It also points out that Spain tends to be left out of discussions of seventeenth-century political thought, arguing that the Spanish monarchy’s context is especially valuable for advancing the historiography on the interaction between political ideas and concrete contexts.