This book is about the mental lexicon and opens an understanding of this aspect of human cognition. The mental lexicon is still a central topic in psycholinguistics and, more generally speaking, in cognitive science. Is it possible to define what is intended by the expression "mental lexicon", a concept coined by Oldfield as early as 1966? Are the terms that the authors have at their disposal still sufficient to discuss this hypothesised mental entity -- the mental lexicon -- which is intended to cover many different aspects of words? The authors propose as a working definition that the mental lexicon corresponds to the mental repository of all representations that are intrinsically related to words. This book extends its research in psycholinguistics and focuses on the word.