ALMOST 50 YEARS HAVE PASSED SINCE THE PUBLICAtion of ASTM STP 169A, in which this chapter was first published. Katherine Mather, who wrote the first two versions of this chapter in ASTM STP 169A and 169B, quoted, in her closing, from St. Paul, “Things which are seen”—concrete and mortar—“were not made of things which do appear.” But then, St. Paul was not a petrographer. St. Paul is not with us—neither is Katherine, who died in 1991. Much of this paper is still her “quote” on petrography, but with changes and additions that update the subject.