Composite membranes represent an emerging and quickly developing class of membrane materials that improve the performance of conventional membranes by focusing on the improvement throughout the membranes without adversely affecting the separation characteristics of the membrane. In this chapter, the various synthesis techniques for the development of multi-layer composite (MLC) membranes as well as the nanofiller particles embedded in the membranes are discussed in detail, with an emphasis on the drawbacks and advantages of all the fabrication techniques. Furthermore, the characterization techniques used most commonly for the analysis of both MLC membranes and nanofiller particles are also explored. Emphasis has been laid on the latest developments in all the synthesis and characterization techniques for MLC membranes as well as nanofillers. Finally, the methods for testing the gas separation performance of MLC membranes are outlined in detail. The working principles of single and mixed gas permeation tests as well as gas chromatography-mass spectrometry (GC–MS) are also discussed, along with their individual benefits and drawbacks. Furthermore, other techniques used for accurate representation of gas separation properties are also described in this work. In essence, this work provides a pathway for the preparation of nanofiller particles and MLC membranes for a number of applications, keeping in view, the need to enhance the performance of existing membrane materials to the maximum possible extent. Therefore, this work can serve as provide a framework to develop membranes for exceptional performance in terms of selectivity and permeability.