Filamentous fungi are important in medicine, industry, agriculture, and basic biological research. For example, some fungal species are pathogenic to humans, whereas others produce β-lactam antibiotics (penicillin and cephalosporin). Industrial strains produce large amounts of enzymes, such as glucosamylose and proteases, and low molecular weight compounds, such as citric acid. The largest and most economically important group of plant pathogens are fungi. Several fungal species have biological properties and genetic systems that make them ideally suited for basic biological research. Recently developed techniques for genetic engineering of filamentous fungi make it possible to alter their detrimental and beneficial activities in novel ways.