The book generalises earlier theories of belief change to cover all kinds of changes of sets by sets. The principal focus is still on changes of belief sets in response to new evidence, but the formal theory extends to all domains with a closure operation and a preference structure including, for example, systems of action. Contraction is the key tion; all other changes can be defined. Various new applications of the theory are outlined. A sentential version of contraction, subtraction, is proposed as a formal counterpart to 'except'-locutions in natural language. Connections are emphasised with other areas at the interface between philosophical logic and artificial intelligence such as reasoning from default assumptions or from inconsistent premises. A relation of merge inference is proposed as a means of retrieving maximal but ntrivial information from inconsistent premises. Merge inference respects certain anti-Boolean intuitions while avoiding a revision of classical logic.