A review. The chlorofluorocarbon (CFC) was a commonly used propellant in metered dose inhalers (MDI), but it has been gradually replaced by hydrofluoroalkane (HFA) in recent years. The hydrophilicity of HFA is stronger than CFC, so that the moisture ingress into HFA-MDI during the preparation and storage process is easier than CFC-MDI. However, the moisture content in MDI, especially in suspension MDI, might affect the product quality. The mechanism of moisture ingress into the MDI is discussed in this paper, and the influence factors on moisture ingress into MDI are analyzed. The European and USA regulatory guidance on moisture in MDI were briefly introduced. The effects of moisture on MDI pharmaceutical performance and the measures to reduce the moisture content in MDI were summarized.