Classical and Quantum W-Gravity

Classical W-gravities and the corresponding quantum theories are reviewed. W-gravities are higher-spin gauge theories in two dimensions whose gauge algebras are W-algebras. The geometrical structure of classical W-gravity is investigated, leading to surprising connections with self-dual geometry. The anomalies that arise in quantum W-gravity are discussed, with particular attention to the new types of anomalies that arise for non-linearly realised symmetries and to the relation between path-integral anomalies and non-closure of the quantum current algebra. Models in which all anomalies are cancelled by ghost contributions lead to new generalisations of string theories.