Hong Kong's coastal waters afford a marginal environment for coral reef growth, with high seasonal and short-term variability in water temperatures (ranging from < 14 degrees C in winter to 31 degrees C in summer), and low summer salinity (as low as 15 psu) due to runoff associated with the Asian wet monsoon season and the Pearl River Delta. Yet Hong Kong hosts 84 reef-building coral species in 28 genera of 12 families of the Scleractinia, distributed in 5 broad communities with strong geographic and environmental affinities and key indicator species. Coral communities farthest from the influence of the Pearl River Delta have relatively high sea bed coverage and species diversity (30 to 50% coverage, and > 30 spp. per site), and also host some large, old corals. X-radiographs of a core of one massive Porites colony confirms similar to 200 yr of growth, with 2 short-term periods of mortality, hidden in the skeleton by overgrowth. Very low linear extension rates (< 4 mm yr(-1)) in this coral and 2 others are anomalous for Porites corals and are indicative of the high-stress environment. Low growth rates suggest that other Hong Kong corals, despite the harsh environmental conditions, may also live for centuries, contributing to the development of 'incipient reefs'.