In the past decades, gas separation using polymeric membranes has received considerable attention and become one of the fastest growing research areas. However, existing polymeric membranes may not be able to keep up with the increasing separation needs for challenging gas mixtures such as N2/CH4 and light olefin/paraffin pairs on industrial scale due to their so-called permeability-selectivity bound. On the other hand, scaling-up issues poise huge challenges for highly permeable and highly selective inorganic membranes. Mixed-matrix membranes, composite membranes, provide an evolutionary solution to debottleneck the permeability-selectivity and scale-up issues currently faced by polymeric and inorganic membranes, respectively. Inorganic fillers in mixed-matrix membranes improve gas permeability and/or selectivity, outperforming polymeric membranes. Combined with relatively economical and simple scaling-up compared to inorganic membranes, mixed-matrix membranes could potentially be a next-generation membrane concept for gas separation applications. This review provides a brief summary on the recent progress in both flat sheet and hollow fiber mixed-matrix membranes with an emphasis on those made over the last five years. A separate section is dedicated to discussing engineering challenges transitioning from laboratory-scale to large-scale synthesis of mixed-matrix membranes. Finally, future prospects and perspectives in mixed-matrix membranes research are briefly outlined.