The most important house dust mites are Dermatophagoides pteronyssinus and in drier areas D. farinae. In subtropical and tropical regions the glycyphagid mite Blomia tropicalis is a major source of allergen, which co-exists with D. pteronyssinus. The group 1 and 2 allergens of Dermatophagoides mites are clearly major specificities and it is likely that these allergens could be the basis of new strategies of immunotherapy for many mite-allergic subjects. About 20% of patients, however, do not have IgE antibody to the group 1 and 2 allergens, and even though this is a minority, it constitutes a large population. There are also many other house dust mite allergens which have high IgE binding activity but these are present in low and variable concentrations in mite extracts, usually at less than 1% of the group 1 and 2 allergens. It must be appreciated that mite extracts are arbitrary preparations that do not accurately represent the relative concentrations of allergens in inhaled air. There is now the opportunity to produce more representative and more balanced formulations of allergens, possibly by mixtures of recombinant allergens. It is likely that the group 3, 5, 7 and 9 allergens will be important along with the high molecular weight group 11, 14, 15 and 18. The tropomyosin group 10 may be an important cross-reacting allergen. B. tropicalis is, because of its distribution in highly populated regions with increasing affluence, a very important allergen. It has low-grade cross-reactivity with Dermatophagoides but most allergens only have 30-40% sequence identity between the different families so they require different allergens for immunotherapy and new diagnostic measures are required to distinguish the sensitivity between the mite families. Studies on B. tropicalis allergens are required to identify the major allergens that do not appear to be the group land 2 specificities. Component resolved diagnosis is a newly developing procedure that uses allergen arrays to provide a diagnostic format to differentiate between cross-reacting allergens and to identify the optimal formulation of allergens for different patients.