The essays collected in this volume, most previously unpublished, address a number of closely interconnected issues raised by the comparative syntax of functional heads within the Principles and Parameters approach. The general theory of head movement, the properties of derived structures created by incorporation, and the parameterization involved are the main theoretical foci. One major empirical area which is addressed concerns head movement in configurations involving certain kinds of operator-like elements, for example, the different manifestations of Verb Second phenomena in Wh and other constructions and the syntax of negative heads and specifiers. In addition, properties of functional heads and head movement in nominal and clausal structures and the causative construction are investigated.