In the early years of generative grammar it was assumed that the appropriate mechanism for generating syntactic structures was a grammar of context-free rewriting rules. The twelve essays in this volume discuss recent challenges to this classical formulation of phrase structure and the alternative conceptions proposed to replace it. Each article approaches this issue from the perspective of a different linguistic framework, such as categorical grammar, government-binding theory, head-driven phrase structure grammar, and tree-adjoining grammar. By contributing to the understanding of the differing assumptions and research strategies of each theory, this volume serves as an important survey of current thinking on the frontier of theoretical and computation linguistics.