This chapter explores the nature of taxonomies in general. It explains six-part taxonomy of the ID knowledge base; and discusses how such a taxonomy can be used by ID scholars and practitioners alike. The chapter explores each theory, its underlying research, and its philosophical orientation. Greenbaum and Falcione's taxonomy consists of major elements, their related major classes, and the related subclasses. Our ID taxonomy is similar to that of Greenbaum and Falcione. Using the Instructional Design Taxonomy to define practice they have defined ID as a very complex process. Learning has been studied almost since the beginning of formal education and yet there is still little consensus as to what it actually is. These different positions result from differing philosophical and theoretical orientations, and different views of what learning outcomes are most important. Not only are there many steps and decisions to be made, but there are many participants, work environments, and resources.