Sewers are known as longitudinal reactors where gases such as methane, nitrous oxide and hydrogen sulphide can be produced. However, gaseous emissions have been mainly assessed in wastewater treatment plants (WWTP). This article presents a critical review of studies that quantify the generation of these gases in sewers and aims to identify the existing research gaps. Differences in sampling methods and site selection, as well as a limited number of studies, result in incoherent comparisons. To address some of these gaps, sampling campaigns were conducted in two Spanish cities. Results showed that wet wells were the most important sources of gases with concentrations up to 321 μg CH4 Lair−1 and 6.8 μg N2O Lair−1. Regarding emission factors, in the case of Calafell, the estimated annual emissions were 18.6 kg CH4 year−1 and 0.3 kg H2S year−1 in summer and 3.8 kg CH4 year−1 and 0.5 kg H2S year−1 in winter. About Betanzos, these values were 24.6 kg CH4 year−1 and 0.5 kg N2O year−1 in summer and 10 kg CH4 year−1 in winter. The summer campaign resulted in greater gas concentration than in the winter season for both cities, suggesting that temperature is a key parameter. We conclude that gas emissions from sewers are significant compared to those of WWTPs resulting in an important contribution to the carbon footprint. Further work needs to be done to assess the gas production along the entire sewer networks, which can result in very different emission factors depending on the sewer components.