The purpose of this review is to evaluate the importance of exposure to microbes in school environments. We summarize the key findings from peer-reviewed literature, describe exposure characteristics, explore the role of these exposures in relation to asthma, allergy and respiratory symptoms, and provide information on the effectiveness of published interventions. PUBMED and Medline databases, local journals, and conference proceedings were searched for studies of both the incidence and prevalence of asthma and allergy symptoms in association with microbes (mould, fungi) in indoor air pollution in schools. We reviewed the literature on indoor microbes and microbe-related respiratory problems in schools and identified commonly reported asthma, allergy, and respiratory symptoms involving schools until 1999. Reported mould and humidity data strongly indicate that endotoxins and moisture are present in many classrooms, possibly leading to health problems. Measurements of airborne bacteria and airborne and surface fungal spores were reported in schoolrooms. Reported microbiological contaminants included allergens in deposited dust, fungi, and bacteria. Levels of endotoxins from bacteria and allergens from fungi or animals were sufficient to cause symptoms in allergic occupants of such classrooms. Effective reduction in bacterial and fungal load should be a major focus of control and intervention efforts.