A newish theory of how to take investment decisions when the future is uncertain, which draws parallels between the real economy and the use and valuation of financial options. It is becoming increasingly fashionable at business schools and even in the boardroom. Traditional investment theory says that when a firm evaluates a proposed project, it should calculate the project’s net present value (NPV) and if it is positive, go ahead. Real options theory assumes that firms also have some choice in when to invest. In other words, the project is like an option: there is an opportunity, but not an obligation, to go ahead with it. As with financial options, the interesting question is when to exercise the option: certainly not when it is out of the money (the cost of investing exceeds the benefit). Financial options should not necessarily be exercised as soon as they are in the money (the benefit from exercising exceeds the cost). It may be better to wait until it is deep in the money (the benefit is far above the cost). Likewise, companies should not necessarily invest as soon as a project has a positive NPV. It may pay to wait. Most firms’ investment opportunities have embedded in them many managerial options. For instance, consider an oil company whose bosses think they have discovered an oil field, but they are uncertain about how much oil it contains and what the price of oil will be once they start to pump. Option one: to buy or lease the land and explore? Option two: if they find oil, to start to pump? Whether to exercise these options will depend on the oil price and what it is likely to do in future. Because oil prices are highly volatile, it might not make sense to go ahead with production until the oil price is far above the price at which traditional investment theory would say that the NPV is positive and give the investment the green light. Options on real assets behave rather like financial options (a share option, say). The similarities are such that they can, at least in theory, be valued according to the same methodology. In the case of the oil company, for instance, the cost of land corresponds to the down-payment on a call (right to buy) option, and the extra investment needed to start production to its strike price (the money that must be paid if the option is exercised). As with financial options, the longer the option lasts before it expires and the more volatile is the price of the underlying asset (in this case, oil) the more the option is worth. This is the theory. In practice, pricing financial options is often tricky, and valuing real options is harder still.