HPSG is a well-developed, precisely formalized theory of grammar whose architecture is based on the notion of constraint satisfaction. Linguistic objects are modeled as feature structures organized via a system of types and constraint inheritance, drawing key insights from research in object-oriented paradigms. The HPSG community values explicit, large-scale grammar development and explores psycholinguistic models, as well as the development of efficient computational systems for processing natural languages using HPSG grammars.