Rationale: Exposure to indoor NO2 has been implicated as a cause of respiratory symptoms suggestive of asthma but evidence has been inconsistent. Objective: To review the existing evidence and examine the effects of indoor NO2 in adult asthma-related symptoms using data from an adult multi-centre cohort (ECRHS) followed up for 20 years. Methods: Studies on respiratory health and indoor NO2 were systematically reviewed and meta-analyses performed. Cross-sectional analyses within a sub-set of ECRHS participants with indoor NO2 measurements were conducted to assess the associations of asthma severity and wheeze prevalence with NO2. A regression model was developed to predict indoor NO2 for a larger ECRHS sample without indoor NO2 measurements. GEE analyses were conducted to examine the long-term effects of gas cooking and modelled indoor NO2 on wheeze and asthma score. To investigate the effect of gas-generating NO2 peaks on asthma exacerbation a panel study was also piloted using a new-to-market portable NO2 sensor. Main results: The systematic review identified 50 studies, mainly in children. Results of meta-analyses suggested a significant association between 12-month period prevalence of wheeze and indoor NO2. Within ECRHS prevalence of wheeze but not asthma severity was associated with measured indoor NO2. Long-term associations of asthma-related symptoms with predicted indoor NO2 exposure but not gas cooking were significant. Interpreting this is difficult as the latter analyses (gas cooking) included a larger number of centres and some heterogeneity across centres was observed in the analysis on asthma score. Gas appliances, outdoor NO2, monthly temperature and country were the main predictors of indoor NO2. Evaluation of the pilot study recommends better recruitment strategies and independent calibration of NO2 sensor. Conclusions: There is some evidence for a link between indoor NO2 and asthma-related symptoms. Health risks may be small but are applied to a substantial proportion of the population.