The formulation, testing protocol, lethal effects, sub-lethal effects, ecological relationships and selectivity of microbial pesticides on non-target, beneficial arthropod natural enemies of insect and mite pests are reviewed for viruses, bacteria, fungi and protozoa. It was found that: (1) effects from indirect mortality of natural enemies are probably more significant across all microbial types than are direct mortalities from microbial pesticides; (2) significant levels of direct mortality of beneficial insects can be caused by bacterial and protozoan microbial pesticides; (3) no direct mortality from viruses of pests has been documented for arthropod natural enemies; (4) the direct effect of fungi on arthropod natural enemies has not been well studied and probably has been underestimated; (5) unique standard methods need to be developed for microbial assays and for expressing more exactly the dosage administered to the host or natural enemy.