Asbestos fibres are present everywhere in our environment. A series of questions concerning, for example, their toxicity or their acceptable levels still remain unanswered. The elaboration of an as accurate as possible reference method for the determination of mineral fibres in air which would be sensitive enough for use in environments with a very low level of contamination is thus called for. From a very short survey of the available methods it can be concluded that a transmission electron microscopy (TEM) method fulfils these requirements. This method is very long and expensive and should be used only in those environments where the level of fibres is low, or in complicated situations where a reference method is required. In other types of environments, such as occupational or paraoccupational situations, other less accurate but more rapid and convenient methods may be used. It is stressed that for any of these methods, and especially for the TEM method, a detailed standardization of the procedure is essential. As a scanning electron microscopy (SEM) method has also been considered for monitoring the ambient environment, the characteristics of both methods are compared, illustrated by photomicrographs and discussed. A TEM method is described in detail as follows: sampling (included recommended air volumes for different contaminated areas), sample treatment, mounting of collected fibres on electron microscopy grids, identification and counting, expression of results and detection limit. Finally, this method is applied to two different paraoccupational situations: two buildings insulated with asbestos. It is then compared with other, more simple methods. For the case of the air contaminated by long fibres (mostly crocidolite) the agreement between the different methods is fairly good. However, for the case where the fibres are short (mixture of man-made mineral fibres and chrysotile) this is not true. These differences are discussed and it is concluded that the choice of the method is dependent on the considered situation and may require preliminary investigations.