Almost every sample of house dust investigated contains common environmental allergens, such as those from house dust mite, dog and cat. For individuals who are not sensitised to these allergens, the concentration of allergen in the home is not important. However, allergen exposure may cause acute symptoms in sensitised individuals with asthma and other allergic diseases, such as rhinitis or atopic dermatitis. It is still very unclear how much allergen exposure is a risk factor for the development of sensitisation to different allergens. It is important, therefore, to be able to quantify accurately the amount of allergen present in the indoor environment in order to identify risk factors for exposure, carry out epidemiological studies to investigate exposure-response effects and monitor the progress of allergen reducing regimes. This paper reviews the current techniques that are available for sampling and quantifying indoor allergens.