Decision theory studies what one ought to do when some more or less desirable outcomes of one’s actions depend on external facts about which one is uncertain. Uncertainty is characterized in terms of probabilities, desires are characterized in terms of utilities, and together they determine the expected utility of one’s actions. This course introduces these notions and their mathematical representations, as well as critically reflects on philosophical questions such as whether one should always take the action that maximize one’s expected utility, whether uncertainty is subjective or objective, and why uncertainty does, or ought to, obey the laws of probability.