This chapter introduces the theory of one-person games. These are decisionmaking problems that are sometimes called one-person games against Nature, and because of their non-social properties some theorists do not consider them to be genuine games. The dilemmas discussed in section 2.2, in which Nature plays no part, are certainly not games in the strict sense, because they do not involve interdependent decisions, but they represent a limiting case that serves as a convenient and logical point of departure. In contrast, risky decisions, which will be discussed in sections 2.3 and 2.4, are interdependent in the formal sense, although they involve no social interaction, because Nature functions as an additional player, and some of the techniques that are used to analyse them will reappear in more sophisticated forms in the solution of two-person and multi-person games in later chapters. In particular, the fundamental ideas underlying probability theory and expected utility theory provide essential tools for the solution of more complex games. Section 2.5 will deal with individual decision making under uncertainty rather than risk, and the ideas developed in that section will also reappear in later chapters in which two-person and multi-person games, which invariably involve decisions under uncertainty, are introduced.