Uncertainty in the Common Ground Marie Nilsenova This thesis is about the uncertainties that naturally form a part of human communication. Language as a communication medium is inherently ambiguous. It is generally assumed that its use is made possible by the existence of common ground among communicating participants, but the question of how common ground comes about is usually left aside. Descriptions of natural conversations suggest that in view of the noisiness of the channel, agents constantly employ means to ensure that changes in the mutual context took place, i.e., they attend to a "dialogue about the dialogue". A particular set of communicative acts used on this level is called grounding. Certain grounding acts pose difficulties to existing formalizations of rational communicative behavior, because they come out as improper or irrelevant. This is an undesirable effect considering the fact that they belong to the most commonly performed dialogue acts. To solve the difficulty, we propose an extension of two recent decision-theoretic approaches to communication. We take it that communicating agents base their actions on probabilistic beliefs about what forms the set of mutually believed propositions. During the interpretation process, the relevance of each utterance is evaluated against the interpreter's epistemic state, represented in a Bayesian model. Subsequent belief changes can ensue, serving as the basis for planning further communicative actions. The analysis can account for the relevance of different types of acknowledging acts, and potentially for other problematic phenomena on the semantic-pragmatic interface.