The identification and contextual definition of concepts is the core of knowledge organization. The full expression of comprehension is accomplished through the use of an extension device called the facet. A facet is a category of dimensional characteristics that cross the hierarchical array of concepts to provide extension, or breadth, to the contexts in which they are discovered or expressed in knowledge organization systems. The use of the facet in knowledge organization has a rich history arising in the mid-nineteenth century. As it has matured through more than a century of application, the notion of the facet in knowledge organization has taken on a variety of meanings, from that of simple categories used in web search engines to the more sophisticated idea of intersecting dimensions of knowledge. This book describes the state of the art of the understanding of facets in knowledge organization today.