This book is intended to be a rather exhaustive research monograph on fuzzy set theory and its applications. The work is based on a large compilation of the literature in English, French, German and Arab. It is not intended here to embed fuzzy set theory in a pure mathematics framework. Sophisticated formalisms, such as that of category theory, do not seem suitable in working with concepts at an early stage of their development. This book is a structured synthesis in an attempt to unity existing works. Such an attempt is made necessary because several search directions have been investigated. Often independently.