This book examines one of the allegedly unique features of human language: structure sensitivity. Its point of departure is the distinction between content and structural units, which are defined in psycholinguistic terms. The focus of the book is on structural representations, in particular their hierarchicalness and their branching direction. Structural representations reach variable levels of activation and are therefore gradient in nature. Their variable strength is claimed to account for numerous effects including differences between individual analytical levels, differences between languages as well as pathways of language acquisition and breakdown. English is found to be consistent in its branching direction and to have evolved its branching direction in line with the cross-level harmony constraint. Structure sensitivity is argued to be highly variable both within and across languages and consequently an unlikely candidate for a defining property of human language.