This chapter provides an overview of both conventional membrane process technologies and optimization of membrane process technologies for CO2 separation from natural gas. Among other natural gas processing operations, removal of CO2 is required to meet gas specification, typically 2% CO2 for pipeline quality gas. The use of CO2-selective membrane technologies for bulk separation of methane is increasing in the natural gas industry. CO2-selective membranes represent a more efficient alternative for CO2 separation, as in this case the separation does not undergo any phase change. The economics of the process, simple operation, and the use of compact modules have led to an increasing exploration of membrane technology for CO2 separation in the natural gas industry over competing separation technologies. This chapter presents an overview of different CO2-selective membranes for the separation of CO2 from natural gas. In particular, the recent significant advances (from 2010) reported in the literature on various CO2-selective membranes, their stability, the effect of different parameters on the performance of the membranes, the relationships between structure and permeation properties, and the transport mechanism applied in different CO2-selective membranes are summarized. Finally, the future direction for CO2-selective membranes is suggested.