Recently, we were asked by the Wiley Publishing Company to write survey articles covering some of the important concepts in operating systems for their forthcoming Handbook of Electrical and Computer Engineering. The three brief articles that comprise this report are the results of those efforts. The first was written by Schneider, the latter two by Andrews. We have collected them here in the hope that some may find them of interest. The first article discusses concurrent programming, which is an underlying concern in most operating systems. The use of a kernel or nucleus in constructing an operating system is described in the second article. Lastly, in the third article, the notion of hierarchical organization is developed and some of the advantage of this approach are presented.