Abstract Devonian (Givetian and Frasnian) reef reservoirs in Alberta and British Columbia contain 60% of the conventional, recoverable oil and 20% of the recoverable gas in the Western Canada Sedimentary Basin. Although the depositional history of these reefs is well understood, it is the diagenetic “overprint” that is often responsible for their reservoir quality. This paper discusses diagenetic facies common to many Devonian reefs and assesses the role of cementation in controlling reservoir quality. Frasnian (Woodbend and Beaverhill Lake Group) reefs are characterized by stromatoporoid and coral knoll reef belts deposited near moderately sloping bank edges. Bank interiors are commonly extensive and characterized by cyclic deposition of grainstone shoals and lagoonal and tidal flat sediments. Certain Givetian reefs found in evaporite basins usually occur as areally small, “pinnacle” reefs with steep (>20°) margins and only minor bank interior development. Givetian reefs studied include the “Presqu'ile” reef complex in northeast British Columbia and Rainbow Member “pinnacle” reefs in northwest Alberta. Frasinian reefs examined in Alberta include: (1) Swan Hills, Judy Creek, Carson Creek, North (Beaverhill Lake Group); (2) Golden Spike, Redwater, Strachan, Ricinus (Woodbend Group); and (3) Nisku reefs at West Pembina (Winterburn Group). Most reefs have been subjected to diagenesis in essentially three environments: (1) submarine (syndepositional, marine to hypersaline pore waters); (2) subaerial (fresh to marine pore waters); and (3) subsurface (well below phreatic aquifers, saline to brackish pore waters). Fibrous and bladed calcite cements, submarine “cracks”, micrite cements, and bored hardgrounds are typical submarine diagenetic fabrics, particularly at bank margins. Submarine cements in reef interiors occur as micro-crystalline or finely crystalline, fibrous, or drusy calcite rims on carbonate grains or as isopachous linings of fenestral pores. Subaerial disconformities are common in most reefs, and associated vadose diagenesis produced localized paleosols, micro-stalactitic cements, and zones with abundant solution porosity. Phreatic cements are predominantly non-ferroan, clear spar calcite with varied Fe2+ and Mn:+ concentrations. Subsurface cementation produced ferroan and non-ferroan calcites and dolomites which in many cases can be related to stylolite formation. Other subsurface diagenesis includes dolomite and anhydrite replacement, sulfide mineralization, and bitumen formation. The major diagenetic influences on limestone porosity are cementation and solution. Submarine cements are commonly extensive in reef margins and in Golden Spike and Rainbow reefs accounting for over 70% of the total cement volume. As submarine cements decrease in importance, porosity generally improves. Alternating cementation and solution associated with fresh water vadose/phreatic environments and localized cementation associated with stylolites is characteristic of most reef interiors. These processes enhance depositionally-controlled layering and produce interbedded porous and well cemented units. This type of “diagenetic stratigraphy” results in stratified reservoirs with numerous permeability barriers which can seriously hamper hydrocarbon recovery. In the subsurface environment, chemical compaction (pressure solution) and associated calcite and minor dolomite cementation are the most important processes affecting reservoir quality. In Swan Hills and “Presqu'ile” reefs, subsurface cements account for over 55% of the total cement volume. In contrast, subsurface cements represent less than 15% of the total cement volume in Rainbow reefs and certain Leduc reefs (Golden Spike, Redwater). The relative lack of subsurface cements helps to preserve reef porosity that survived early cementation.