Megachilidae is a cosmopolitan family of mostly solitary bees. Characteristic traits of this family are the restriction of their pollen-carrying structure to the ventral surface of the abdomen, and their typically elongated labrum. Megachilid genera are most commonly known as mason bees and leafcutter bees, reflecting the materials from which they build their nest cells ; a few collect plant or animal hairs and fibers, and are called carder bees, while others use plant resins in nest construction and are correspondingly called resin bees. All species feed on nectar and pollen, but a few are kleptoparasites, feeding on pollen collected by other megachilid bees. Parasitic species do not possess scopae. The motion of Megachilidae in the reproductive structures of flowers is energetic and swimming-like; this agitation releases large amounts of pollen.

Mason bee is a name now commonly used for species of bees in the genus Osmia, of the family Megachilidae. Mason bees are named for their habit of using mud or other "masonry" products in constructing their nests, which are made in naturally occurring gaps such as between cracks in stones or other small dark cavities. When available, some species preferentially use hollow stems or holes in wood made by wood-boring insects.