  This paper discusses how a measure of uncertainty representing a state of
knowledge can be updated when a new information, which may be pervaded with
uncertainty, becomes available. This problem is considered in various
framework, namely: Shafer's evidence theory, Zadeh's possibility theory,
Spohn's theory of epistemic states. In the two first cases, analogues of
Jeffrey's rule of conditioning are introduced and discussed. The relations
between Spohn's model and possibility theory are emphasized and Spohn's
updating rule is contrasted with the Jeffrey-like rule of conditioning in
possibility theory. Recent results by Shenoy on the combination of ordinal
conditional functions are reinterpreted in the language of possibility theory.
It is shown that Shenoy's combination rule has a well-known possibilistic
counterpart.
