Aspergillus fumigatus and related species are widely distributed in nature. The majority of the species belonging to this genus are saprophytic in nature. Only a few species including A. fumigatus are capable of causing diseases in man. These opportunistic agents may cause infection or allergy in susceptible individuals. These fungi also cause diseases in animals, birds, and in plants. In addition, some of the enzymes and metabolic products have tremendous value in industry. A few of the Aspergillus species produce potent toxins of the aflatoxin family, which can cause cancer. Toxic death due to aflatoxins has been reported in humans, animals, and birds. There are currently about 180 recognized species of Aspergillus, and these species are placed in 6 subgenera, which are further divided into several sections. The telemorphs belonging to the genera are Chaetosartorya, Dichlanea, Eurotium, Emericella, Fennellia, Hemicarpentales, Neosartorya, Petromyces, Sclercleisia, and Warcupiella. This review presents a concise overview of the ecology, taxonomy, and genetics of Aspergillus species including their role in plant, animal, and human diseases, production of toxic metabolites, and molecular methods for their identification.