Questionnaires are the most used subjective instrument of measurement in respiratory epidemiology. The standardisation of the questionnaires aims to limit bias by maximising validity and reliability, and comparability. Within the European Union project BIOMED1, a compendium of respiratory standard questionnaires (CORSQ) was developed for adults covering 18 topics from general information to early life events, through environmental risk factors and respiratory symptoms and diseases. Reliable spirometry data needs a rigorous quality control programme, as in the "Salute Respiratoria nell'Anziano" (Sa.R.A.) project, Italian for "Respiratory Health in the Elderly". Reproducibility rates were 95.8% for forced expiratory volume in one second (FEV1). Male sex and age were independent risk factors for a poorer reproducibility, as well as cognitive and physical impairment (shorter 6-min walking distance) and lower educational level for a poorer acceptability. Reference values for people aged 65-85 yrs have been produced; these results suggest that the effect of aging should be corrected for physical and mental disability. A revision of interpretative strategies included in current guidelines is needed. Peak expiratory flow monitoring has several methodological problems: reliability and sensitivity of the measurement in order to detect changes in airway calibre; compliance with long-term monitoring; choice of the best variability index; difference between asthmatic and nonasthmatic subjects; age-related differences. Despite these methodological problems, peak expiratory flow monitoring has been successfully used in the evaluation of the effects of air pollution in normal and asthmatic subjects, and in the elderly.