A recently discovered Silurian reef tract in the high Canadian Arctic has an exposed length of more than 150 km and is locally more than 500 m thick. The reefs occur within a Silurian ramp and rimmed shelf-margin sequence about 2 km thick. Conodont biostratigraphic data indicate that the reefal strata are late Llandovery to Ludlow in age, and reefs are composed largely of coral-microbial boundstone, The corals are mainly digitate rugosans and large halysitids, associated with fewer stromatoporoids and lithistid sponges. However, most of the reefal strata consist of microbialite, which encrusts skeletal metazoans or is massive or thromboidal. Microbialite microstructure consists mainly of clotted micrite or laminated micrite associated with common Renalcis, other calcimicrobes, and locally abundant early marine cements. Other reef rock types include stromatactis-rich lime mudstone, cementstone, zebroid stromatactis-bearing lime mud-stone, crinoidal grainstone, and, rarely, stromatoporoid boundstone. The reefs formed on the shelf margin and prograded basinward in three distinct phases during Wenlock and Ludlow time. Underlying Llandovery, shelf-margin facies are highly dolomitized and may represent an original stromatoporoid boundstone. Overlying late Ludlow to Pridoli carbonates were deposited on a prograding carbonate ramp, dotted with small coral-stromatoporoid and other biostromes, Three periods of Silurian reef growth can be recognized in the Arctic: Llandovery, latest Llandovery-late Ludlow, and late Ludlow-Pridoli. The Llandovery phase was widespread, and large stromatoporoid-coral reefs occupied a variety of depositional settings, including shelf-edge, intrashelf, drowned shelf, and slope. Late Llandovery to late Ludlow reefs were also regionally extensive, but were predominantly coral-microbial boundstone and locally grainstone-rich structures containing accessory metazoan (stromatoporoid-coral) framestone and boundstone. These formed high-relief structures with coarse allochthonous debris aprons. Large stromatoporoid-coral reefs were also present, but these were confined to areas characterized by high rates of siliciclastic sedimentation or to intrashelf areas. Another regional change in reef composition occurred in the latest Ludlow. Initially, reefs were rare and dominated by lithistid sponges and microbialites, but in Pridoli time, they became widespread and were constructed by corals and stromatoporoids, as were their Llandovery ancestors. In addition, the Pridoli reefs did not attain the large sizes of reefs in the two older sequences. The three phases of reef development can be related to important, basinwide sequence boundaries. Microbialite-rich reefs have also been described from northwestern Russia, Alaska, and eastern Canada and are some of the largest reef structures known. Their large size reveals a need to reconsider popular reef evolutionary models, in which skeletal metazoans are regarded as the predominant constructors. Large reef size is implied in these evolutionary models, but, based on this and other reef studies, microbialite-rich reefs equal, and commonly surpass, the dimensions exhibited by coral-stromatoporoid reefs. Considering the microbialite-rich reefs, now known from both the Ordovician acid Silurian, it is clear that benthic microbial communities played a very important role in reef construction through the lower Paleozoic.