The genus Leioseius Berlese is reviewed, faunistically in North America, and taxonomically, world-wide. Descriptions of two new species, redescriptions of two previously recorded species, and first records of Leioseius elongatus Evans in North America are presented. Observations indicate that various leg setal and pretarsal attributes are useful in taxonomic discrimination of species in this genus. A key to North American species based on both idiosomatic and leg setal aspects is presented. Four species previously described under the genus Iphidozercon Berlese are accounted for the first time as species of Leioseius, and other taxa described previously as four yet other species of Leioseius are transferred to other genera. A tentative key, provided for separation of females of the 22 recognizable world Leioseius species, accounts for these changes. General morphological notes about mites of this genus are presented, some of which may lead to recognition of species-groups. Scant, general biological aspects about Leioseius mites are presented.