Introduction The evolutionary classification for coral reefs of Hopley (1982), although applied elsewhere, was initially devised specifically for the reefs of the Great Barrier Reef (GBR). The classification (Figs. 5.6 and 5.7, Table 5.3) shows antecedent platforms being recolonized, with reef growth during initial juvenile stages being largely vertical, enhancing the original relief of the antecedent surface. A mature phase after reefs reach or catch up with sea level follows, during which both growth and sedimentation infill and obliterate the previously constructed relief. The senile phase consists of flat-topped or planar reefs. At the time the classification was devised there were few drill holes which had penetrated the Holocene reef on the GBR and great reliance was put on seismic refraction survey to identify the depth of the Holocene (Harvey, 1977a, b, 1980; Harvey et al ., 1979) and approximately 102 radiocarbon dates from surface or near-surface samples from 24 reefs (Hopley, 1982, table 9.2). A number of intuitive presumptions were also made at this time and these formed many of the questions asked by the drilling projects on the GBR subsequent to about 1980. These studies have built up a considerable data bank on the age and structure of mid-shelf reefs (Table 8.1). Outer shelf or shelf marginal reefs such as the ribbons of the northern GBR, some of the outermost reefs of the central GBR, and the unique Pompey Reefs of the stepped shelf margin of the south central GBR are discussed in Chapter 9 whilst Chapter 11 synthesizes the structural and growth data of all reefs, fringing, mid-shelf, and outer shelf.