This paper discusses the use of the term “hierarchically structured” to describe the design of operating systems. Although the various uses of this term are often considered to the be closely related, close examination of the use of the term shows that it has a number of quite different meanings. For example, one can find two different senses of “hierarchy” in a single operating system [3] and [6]. An understanding of the different meanings of the term is essential, if a designer wishes to apply recent work in Software Engineering and Design Methodology. This paper attempts to provide such an understanding.