 Richard Jeffrey’s expected utility theory differs from Savage’s in terms of both the prospects (i.e., options) under consideration and the rationality constraints on preferences over these prospects. The distinct advantage of Jeffrey’s theory is that real-world decision problems can be modelled just as the agent perceives them; the plausibility of the rationality constraints on preference do not depend on decision problems being modelled in a particular way. We first describe the prospects or decision set-up and the resultant expected utility rule, before turning to the pertinent rationality constraints on preferences and the corresponding theorem.