THIS CHAPTER CONTINUES THE TRADITION OF previous editions of ASTM STP 169. Due to the fundamental nature of these topics, much of what is here has appeared before in ASTM STP 169B, authored by F. F. Bartel, and ASTM STP 169C, authored by the present author. This chapter updates the language and expands on the utility of density measurement in conjunction with air content measurement to improve test reliability. The author would also like to acknowledge the support and encouragement of his company and the many colleagues who have provided useful insight.