Moulds are responsible for diseases in humans through the three pathogenetic mechanisms of infection, allergy, and toxicity. Fungal infection is especially a risk factor for immunodeficient patients, but it occurs in immunocompetent patients as well. Fungal allergy is manifested as bronchial asthma, hypersensitivity pneumonitis, allergic bronchopulmonary aspergillosis, or allergic fungal sinusitis. Mycotoxicosis is almost exclusively the result of ingestion of mould-contaminated foodstuffs. In each case there is specificity for the etiologic mould. There is controversy regarding the ability of indoor airborne mould spores to cause human disease through non-specific toxicity via the inhalation route. Pulmonary mycotoxicosis is an established, although rare, occupational disease of farmers who inhale enormous quantities of mycotoxins, endotoxins, and other toxic chemicals from contaminated silage. Other conditions attributed to indoor airborne mycotoxin are unproven. These include infantile pulmonary hemosiderosis, epistaxis, 'toxic encephalopathy', immune dysregulation and a variety of subjective complaints without objective signs of pathology such as fatigue, headache, dyspnea, gastrointestinal distress, neuromuscular and skeletal complaints, etc. Non-specific irritation from moulds via the inhalation route is also a controversial subject that remains unproven. Published studies alleging an epidemiologic causal relationship are unconvincing.