We present an interactive system organized around networks of constraints rather than the programs whlch manipulate them. We describe a language of hierarchical constraint networks. We descrlbe one method of deriving useful consequences of a set of constraints whlch we all propagation. Dependency analysis is used to spot and track down inconsistent subsets of a constraint set. Propagation of constraints is most flexible and useful when coupled with the abi1ty to perform symbolic manipulations on algebraic expressions. Such manipulatons are in turn best expressed as alterations or augmentations of the constraint network. Numerous diagrams ornament the text.