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The use of peak flow meters has been widely adopted for monitoring patients with asthma. The Working Party of the European Respiratory Society (ERS) has solely addressed technical and physiological issues relating to peak expiratory flow (PEF) (flow describes the rate of change of volume (volume rate), so that flow rate is equivalent to volume acceleration. Hence, PEF should be used in preference to peak expiratory flow rate (PEFR)). Monitoring schemes, or comparison of PEF with other indices, such as the forced expiratory volume in one second (FEV1), do not form part of these recommendations. Measurements of PEF are of value in identifying airflow limitation. The correlation between airflow and symptoms is variable, some patients being poor perceivers of changes in airway patency, whereas others quickly perceive small changes [1‐6]. Recording the PEF is, therefore, of value in clinical practice where it can be helpful in monitoring the progress of airflow limitation and the effects of treatment, and in epidemiological and occupational studies for identifying the presence of airflow limitation, assessing its severity and variation. Various types of instrument can be used to measure PEF, including pneumotachometers, spirometers, turbines and anemometers. By far the most suitable and commonly used instruments in clinical practice are flow meters which measure PEF only and, hence, may be referred to as peak flow meters. Since they are massproduced, they are relatively inexpensive; furthermore, they are portable and do not require electrical power for their operation. Most handheld peak flow meters employ the principle of a variable orifice to measure airflow indirectly. The pressure exerted by a forced expiration causes a diaphragm or vane to move and, in so doing, to open a progressively larger area of the orifice. The point at which no further movement of the diaphragm occurs depends on the maximal pressure and, hence, on the peak expiratory flow that has been generated.