Occurrences of large-scale, parasite-related fish diseases have increased as the number and size of hatcheries, mass culture facilities, and public and private desplay aquaria have grown. This increase is due to concentration of hosts and parasites in confined quarters resulting in enhanced opportunity for infestations and epizootics. Monogeneans, usually well accommodated to their fish hosts in nature and causing few easily detectable effects, frequently cause severe epizootics in cultured or aquarium-held populations. In such cases, the short, direct, one-host life cycles of monogeneans enable them to reach epizootic levels very quickly when hosts and parasites are confined closely together. Also, despite their high host-specificity in the wild some monogeneans may infest numbers of other hosts when several or many fish species are confined together. Further, debilitating attacks by these helminths may be accompanied by invasions of viruses, bacteria, or fungi. Such invasion is usually via portals of entry caused by trauma of feeding or attachment by the worms. Blood-feeding monogeneans may transmit disease directly. When single or mixed-species host populations are under stress, monogeneans, like other normally benign parasites, may become pathogenic. Stress in hosts results from any single factor, or combinations of factors including: poor sanitation, poor nutrition, poor water quality, overcrowding, behaviorally incompatible species, or improperly designed culture systems or aquaria. Whatever the cause or route of infestation by monogeneans or associated flora and fauna, health and growth of hosts is usually impaired by heavy parasite attack. Deaths are not uncommon. Culturists and aquarists alike must take precautions against monogenean-related disease. Foremost among the precautions is design and thoughtful utilization of adequate holding and display facilities, and adherence to adequate acquisition and introduction procedures, such as the ICES (International Council for the Exploration of the Seas) methods. Careful quarantine and introduction procedures, with thorough monitoring of water quality are paramount. Chemical prophylaxis is possible and should be practiced as needed. When infestations occur, as they often do despite precautions, chemical treatments must be introduced. When epizootics persist, stocks must be harvested quickly or discarded. In aquaria, isolation and treatment of infested individuals or systems may be useful. If this fails, destruction is recommended. The best measure against parasitic monogeneans is avoidance or prevention. Effective sanitation is critical and must be rigorously applied. These and other aspects of monogenean infestation, associated pathology, and prevention or treatment are described and discussed.