Jean-Roger Vergnaud's work on the foundational issues in linguistics has proved influential over the past three decades. At MIT in 1974, Vergnaud (now holder of the Andrew W. Mellon Professorship in Humanities at the University of Southern California) made a proposal in his Ph.D. thesis that has since become, in somewhat modified form, the standard analysis for the derivation of relative clauses. Vergnaud later integrated the proposal within a broader theory of movement and abstract case. These topics have remained central to theoretical linguistics. In this volume, essays by leading theoretical linguists attest to the importance of Jean-Roger Vergnaud's contributions to linguistics. The essays first discuss issues in syntax, documenting important breakthroughs in the development of the principles and parameters framework and including a famous letter (unpublished until recently) from Vergnaud to Noam Chomsky and Howard Lasnik commenting on the first draft of their 1977 paper "Filters and Controls." Vergnaud's writings on phonology (which, the editors write, "take a definite syntactic turn") have also been influential, and the volume concludes with two contributions to that field. The essays, rewarding from both theoretical and empirical perspectives, not only offer insight into Vergnaud's impact on the field but also describe current work on the issues he introduced into the scholarly debate.