This paper addresses the dual problems of recognizing when an agent is expressing an attitude of doubt toward a proposition and of realizing natural language utterances that effectively convey such doubt. It presents naturally occurring examples that illustrate the wide variety of ways in which doubt can be expressed. The paper presents a recognition algorithm that uses linguistic, contextual, and world knowledge to determine when an agent is expressing doubt at a proposition and to identify the proposition that is being doubted. It then presents a realization algorithm that uses these knowledge sources to formulate utterances that will convey doubt when an agent cannot accept a proposition due to conflict between it and some other proposition in the agent's knowledge base.