This book addrsesses the dialog issues that must be resolved in building effective spoken natural language dialog systems. The authors present an architecture for dialog processing for which an implementation has been constructed that exhibits a number of behaviours required for efficient human-machine dialog including: problem-solving to help the user carry out a task, coherent sub-dialog movement during the problem-solving process, user model usage, expectation usage for contextual interpretation and error correction, variable initiative behaviour for interacting with users of differing expertise. Readers should gain an understanding of how to construct a spoken natural language dialog system capable of exhibiting the necessary behaviours for effective human-computer natural language interaction.