 Decision theory is concerned with the reasoning underlying an agent’s choices, whether this is a mundane choice between taking the bus or getting a taxi, or a more far-reaching choice about whether to pursue a demanding political career. (Note that “agent” here stands for an entity, usually an individual person, that is capable of deliberation and action.) Standard thinking is that what an agent chooses to do on any given occasion is completely determined by her beliefs and desires or values, but this is not uncontroversial, as will be noted below. In any case, decision theory is as much a theory of beliefs, desires and other relevant attitudes as it is a theory of choice; what matters is how these various attitudes (call them “preference attitudes”) cohere together.