Aeroallergens play a major role in the pathogenesis of allergic diseases, particularly asthma and rhinitis. Indoor allergens, including house dust mites, domestic pets, cockroaches, and molds are of particular importance. Pollens are also recognized as a major source of allergens. The role of these different allergens varies with environment conditions, such as climatic factors, and degree of exposure. Knowledge about allergens has progressed, especially with recent molecular biology studies. Structure and function have been identified. These studies have provided explanations about the relationship between allergic sensitization, allergen exposure, and disease activity, about clinical observations such as allergic cross reactions, and improvement in the production of allergenic extracts (necessary to diagnosis and immunotherapy). Environmental control measures are of particular importance in the prevention and management of allergic diseases.