Ordovician reefs have a relatively restricted distribution within carbonate depositional belts of the world. Early Ordovician reefs mainly comprise undifferentiated algal- or sponge-dominated mounds of small size. Middle Ordovician reefs have a much broader geographical distribution, a wider range of morphological differentiation and some attain barrier-reef dimensions. A much greater variety of frame-building organisms are represented including stromatoporoids, corals and bryozoans. Late Ordovician reefs are similarly widely distributed but are not as large. Of the two phases of warming suggested by reef occurrences in Baltoscandia, the first in the late Carodoc probably results from a short-lived maximum expansion of the tropical-subtropical belts with accompanying reduction in the latitudinal temperature gradient. The second in the late Ashgill coincides with the period of major glaciation. Reefs continued to grow at the height of the glaciation but apparently in much narrower tropical-subtropical belts.-from Author Dept. Geol. & Geophysics, Univ. of Sydney, NSW, 2006, Australia.