Gas cleaning membranes are the core part of gas–solid or gas–gas separation technology. Compared with traditional filter-based filtration technology, membrane technology offers enhanced energy efficiency, environmentally friendliness, higher efficacy, and broader applicability and have widespread applications in treating industrial exhaust gases, recovering high-value-added powders, maintaining cleanrooms, and purifying indoor air. They have been regarded as critical solutions for addressing major challenges in the fields of energy, environment, and the transformation of traditional industries. The development of gas-cleaning membranes and their technologies currently show the following characteristics. First, gas-cleaning membrane materials are developing towards high removal efficiency, low filtration resistance, and multifunctionality. Second, the market for gas-cleaning membrane materials is rapidly expanding, and its integration with upstream and downstream raw material production and equipment manufacturing industries is becoming increasingly intensified. In the future, the role of gas-cleaning membrane technology in promoting energy conservation and emission reduction, resource recycling, and industrial structure upgrading will become increasingly apparent. Furthermore, as the demand for a healthy and safe living environment continues to rise, the utilization of gas-cleaning membranes is expected to see rapid growth in various civilian applications such as personal protection, household air purifiers, special protective fabrics and equipment, among others. In this Chapter, the definition of gas cleaning membranes and these related terms will be introduced, followed with the emphasize on the principles of gas–solid separation.