Constraint and unification-based approaches to grammar have become increasingly popular in computational linguistics because of their flexibility and descriptive power. These approaches have developed an important notion of feature structures that play a key role in the representation of linguistic information. This book provides a detailed survey and comparison of recent approaches to the logical formalization of feature structures and their description languages in constraint and unification-based grammar formalisms. Bill Keller is a lecturer in computer science and artificial intelligence in the School of Cognitive and Computing Sciences at the University of Sussex.