Although many actions, such as playing or listening to music, may be done just for their own sake, most are not. Most actions, like taking exercise to keep fit, are done partly or mainly as a means to an end, the end being an intended effect of the action. Many actions, indeed, are only done as means to ends, as when we pay money to buy goods. In these cases agents need to decide whether (they think) the end justifies the means, e.g. whether (they think) the goods are worth the price; and the job of decision theorists is to say on what grounds such decisions are or should be made. And so they do; but the grounds that most of them give are wrong, I shall argue, because they are subjective. Merely thinking that an end justifies a means, or fails to justify it, is not enough to justify our adopting, or declining to adopt, that means to that end. But to say this is easier than to say what more it takes to justify making such decisions, and what we should do when, as is often the case, we know too little to justify them. Those are the questions I set out to answer in this chapter.