Since the 17th century, 21 species of foreign bees are known to have joined the North America fauna of 3,800 species, comprising 0.5% of the overall continental fauna. Among these, only the honey bee is social. Among the six deliberately introduced bee species, all but the honey bee were released to pollinate agricultural crops. Most others were transported inadvertently with trans-Atlantic cargoes from the western Palearctic. Many of these foreign bee species remain restricted to limited areas of the eastern United States and adjoining Canada; four species have spread transcontinentally. Nearly all of these exotic species nest in stems or wood; two species nest in the ground; and one constructs free-standing nests of mud and pebbles. Most of these foreign bee species are polylectic (=use a taxonomic diversity of floral hosts for pollen), whereas the four narrowly oligolectic species continue to specialize on the same pollen hosts as in their native Europe. No evidence to date implicates these foreign nonsocial bee species in deleterious exploitative competition with native North American species for nectar or pollen (outside of agricultural crops). However, several investigators suggest that one or more Palearctic species of Megachile in California may be excluding native megachilids from suitable nesting cavities.