Imagine you buy an umbrella. You pay a small amount now, and it only helps you if it rains tomorrow. That’s the basic idea of a state-contingent claim—a payoff that depends on what actually happens. This chapter takes that simple thought and builds a framework that turns every financial asset—stocks, bonds, options—into a bundle of such state-claims. Along the way you will see how markets price risk, why some risks can be perfectly hedged while others cannot, and what the rule “no free lunch” really means.