Technical Abstract: Nesting substrates and construction materials are compared for 65 of North America’s 139 described native species of Osmia bees. Most accounts report them nesting in pre-existing cavities in dead wood or pithy stems such as elderberry, with cell partitions and plugs made from a pulp of finely masticated leaf tissue. Mud is widely used by species constructing free-form clumps of nest cells against stone surfaces. Some Osmia adopt abandoned nests of other Hymenoptera, particularly those of mud dauber wasps (Sceliphron spp.) and larger ground-nesting bees (e.g. Anthophora spp.). Reports of subterranean nesting by Osmia species are uncommon, but probably under-represent the habit, because their subterranean nests are obscure and likely to be scattered. Ground- or surface-nesting habits are suspected for many of those species that are absent from intensive trap-nesting programs in their native ranges, but that otherwise have been taken commonly at flowers.