Abstract Of the approximately 200 Ordovician reefs recorded from carbonate successions worldwide, about half come from the Early-Middle Ordovician (Tremadocian-Darriwilian), and the other half from the Late Ordovician (Caradocian-Ashgillian). Eight distinctive reef-forming phases have been differentiated through the Ordovician Period. The most dramatic event in Ordovician reef development occurs across the Middle-Late Ordovician boundary with the changeover from microbial-dominated to metazoan and algal-dominated reefal growth. Early-Middle Ordovician reefs are predominantly microbially constructed mud mounds that exhibit a variety of macrofabric types, especially thrombolitic (clotted) and stromatolitic (laminar and digitate). Climax of microbial growth occurred during the Tremadocian, in association with kilometer-scale, rimmed, shelf-edge reef complexes, and other large reefs of complex microbial community intergrowths with thrombolitic cores, stromatolitic crusts, and sponges. Microbial reef consortia with sponges (especially siliceous lithistids) and receptaculitalean Calathium commonly occur. Other metazoan components (Iichenariid corals, bryozoans) are localized and rare. Massive skeletons of pulchrilaminid stromatoporoids (calcified sponges) also appeared in a limited frame-building role during the late Early Ordovician. Large stromatactis-bearing mud mounds developed in outer platform to downslope sites of the early Middle Ordovician, perhaps coincidentally as thrombolites began to significantly decline. Also localized, shallow-water mud mounds occur in the early Middle Ordovician of intermediate paleolatitudes in Baltica. With metazoan and algal expansion at the Middle-Late Ordovician boundary, microbial components became less conspicuous, though they continued in construction of carbonate mud mounds and stromatolitic mound complexes. The latest Middle Ordovician (Darriwilian) metazoan expansion is depicted in on-shelf sites of eastern North America by an array of bryozoan-, stromatoporoid-, coral-, and sponge-dominated reefs, and the appearance of cavity-dwelling biotas. Spread of bryozoan-dominated reefs, some large, 150-m-thick, reef complexes, in more offshore, shelf-edge to downslope, habitats, followed during the earliest Late Ordovician (early Caradocian). Middle-late Caradocian reef history is characterized by a wider circumglobal spread of mainly on-shelf reefs with corals and stromatoporoids dominant, but also receptaculitalean, solenoporacean, and dasycladalean algae, as major reef contributors. A 15-m-thick, prograding, labechiid stromatoporoid reef complex in a mid-ramp site of southern Norway exhibits laterally differentiated fore-reef and back-reef fades. Debris-producing algal-coral-stromatoporoid reefs occur on rifted outer-platform margins in North China, and a variety of fringing microbial-algal reefs and an atoll are preserved in volcanic island-arc settings of Kazakhstan. Even wider differentiation and global spread of reefs occurred during the Ashgillian (especially mid-Ashgillian), with corals, stromatoporoids, algae, bryozoans, and microbes contributing to a wide range of on-shelf reefs in platform and island-arc settings. Corals, stromatoporoids, and algae are main contributors to larger, platform-rimmed reef complexes in the Urals. Microbial-dominated pinnacle reefs, some 30 m high, developed in shallow, restricted waters of the intracratonic Foxe and Hudson Bay basins (northeastern Canada). Stromatactis-bearing carbonate mud mounds, up to 150 m thick, formed in deeper, on-shelf sites (Sweden, Baltic Sea), and sometimes nearer to the shelf margin (southeastern China). An allochthonous, kilometer-size, microbe Trichwiophyton-dominated reef in southeastern China probably grew on the adjacent shelf margin, prior to massive slumping. Reefs also developed in intermediate-higher paleolatitudes of north Gondwana. In Spain, deeper, mid-ramp, bryozoan and pelmatozoan mud-mound complexes occur, and in North Africa (Libya), only about 2000 km from the Ordovician south paleopole, shallow, 100-m-thick, bryozoan-dominated biostromes. Only a few geographically restricted, low-paleolatitude regions exhibit late Ashgillian (Himantian) reefs, but they are diverse, coral-dominated, and appear to have grown during the mid-Hirnantian interglacial phase, between base- and late Himantian glacial pulses. A review of factors that may have influenced the major changes in Ordovician reef development is presented against an earth-system background of profound global and biotal change that includes a greenhouse climate, a sluggish, saline oceanic state, high sea levels, progressive ventilation of surface waters, intense episodes of volcanicity and orogeny, massive diversification of biotas, and an apparently short, sharp, end-Ordovician glaciation with accompanying extinction. Major controls on reef growth are inferred to include: (1) under the existing high levels of atmospheric carbon dioxide of the Early Ordovician, higher-than-normal temperatures promoting microbial reefal communities and cyanobacterial calcification processes—high temperatures may have exceeded adaptive ranges of many metazoans (excepting sponges), delaying their proliferation as reef builders; (2) sea-level fluctuations and elevated sea-level highstands of the Early Ordovician caused episodic drowning and influx of anoxic waters from an expanded oxygen-minimum zone over platforms and archipelagos, limiting reefal diversification to more rudimentary phases involving mainly microbial growth; (3) lower sea-level highstands during the Middle Ordovician restricted flooding to outer shelves, and consequently reef building occurred only in peripheral parts of platform areas, hence the lower abundances (only 18% of all preserved Ordovician reefs); (4) apparent rise in levels of oxygenation of tropical surface waters to a peak near the Middle-Late Ordovician boundary aided significantly the colonization of the reef habitat by sessile, respiring metazoans; and (5) elevated Late Ordovician sea-level highstands again causing episodic drowning over platforms, but this time influxes of oceanic waters had a limited, mainly regional, impact on reef development. Diversification and spread of reefs continued through the Ashgillian but, in the lead up to the end-Ordovician glaciation, late Ashgillian (Hirnantian) reefs developed in only a few sites, though they retained diverse biotas.