IN PREPARATION OF THIS CHAPTER THE CONTENTS of the 4th edition were drawn upon, as the 4th edition had drawn upon Samuel Helms's clear and concise work in the previous three editions of this ASTM Special Technical Publication. The discussion of air content is more extensive here, however, largely in response to the continuing interest and research in air-entrained concrete and freeze-thaw durability. It has also become evident that the further in time that the industry has come from the landmark contributions of the early workers in air-void systems and frost resistance in concrete, the less well understood are the fundamental principles laid down in the late 1940s and early 1950s. Following the pattern set by Helms, the present edition therefore reviews much of this key literature to explain the origins of today's state of the art. Finally, this edition appears when there is a proliferation of the use of the ASTM Practice for Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete (C 457) microscopical analysis procedure, which was once exclusively performed by a small cadre of experts. While expansion of the use of the test has produced more data, it has in some cases produced more confusion where less experienced operators have used nonstandard equipment and procedures. The chapter has therefore been written in part to point out pitfalls and sources of error in performing and interpreting the test.