This volume contains revised and expanded versions of those papers from the 1990 Functional Grammar Conference in Copenhagen that contributed specifically to the current investigation of clause structure in terms of semantic layers. One of the key concepts in this discussion is 'reference'. Some papers discuss ways in which previous accounts of reference need to be expanded and differentiated to provide a consistent picture of referential properties. The power of layered analysis to bring out fundamental similarities between languages of very different types is the theme of another group of papers, again with the referential properties of constituents playing a central role. By some contributors layered analysis is challenged, and the question is raised as to how it might fit into a dynamic and pragmatic picture of language. The book is rounded off by a comparison between layered structure in Functional Grammar and in Government and Binding Theory.