The morphology of the medically important house-dust mites Dermatophagoides pteronyssinus (Trouessart), D. farinae (Hughes), Euroglyphus maynei (Cooreman) and E. longior (Trouessart) has been comprehensively reviewed. A taxonomical key for all four species was constructed and elaborated with drawings and scanning electron micrographs. Biological studies indicated significant differences occurring between these species, with respect to their pre-reproductive period, reproductive period, fecundity and development time.