While it has long been taken for granted that context or background information plays a crucial role in reference assignment, there have been very few serious attempts to investigate exactly how they are used. This study provides an answer to the question through an extensive analysis of cases of bridging. The book demonstrates that when encountering a referring expression, the hearer is able to choose a set of contextual assumptions intended by the speaker in a principled way, out of all the assumptions possibly available to him. It claims more specifically that the use of context, as well as the assignment of referent, is governed by a single pragmatic principle, namely, the principle of relevance, which is also a single principle governing overall utterance interpretation. The explanatory power of the criterion based on the principle of relevance is tested against the two major, current alternatives - truth-based criteria and coherence-based criteria - using data elicited in a battery of referent assignment questionnaires. The results show clearly that the relevance-based criterion has more predictive power to handle a wider range of examples than any other existing criterion. As such, this work adds to the growing body of evidence supporting the insights of relevance theory.