Twenty-seven years have past since the publication of ASTM STP 169A, where this chapter was first published. Kay Mather, who wrote the previous version of this chapter in ASTM STP 169A and 169B, in her closure quoted from St. Paul, “Things which are seen”—concrete and mortar—“were not made of things which do appear.” St. Paul is not with us—neither is Kay who died in 1991. The bulk of this paper is still her “quote” on petrography, but with modest changes and additions. The next version may be entirely different. But the emphasis will remain the same.