Fungi are important pathogens of plants and cause more significant yield losses than bacteria or viruses. However, bacteria and viruses are more important than fungi as pathogens of animals; indeed, whether or not a fungus even becomes pathogenic on an animal often depends on the immune status of the host. Until the rapid rise of opportunistic fungal infections in humans, pathogenicity mechanisms in plant pathogens were better understood than those in animal pathogens. Increased research activity in medical mycology has coincided with the development of molecular genetic and genomic resources, which are being exploited to develop a detailed understanding of fungal pathogenesis in both animals and plants. Several constraints and peculiarities govern the types of information that can be derived from such studies. For instance, analyses of human-pathogenic fungi generally rely on cell lines and experimental animal models, in contrast to plant pathogens, which can be studied directly on their hosts. Many more fungal species infect plants than animals, and thus, more plantfungus systems than animal-fungus systems are studied. This is mainly because there are far more plant hosts than animal hosts that are of economic importance, with the obvious exception of human disease, and because plants can be manipulated without the ethical issues associated with animal experimentation.