Abstract We have reviewed the development of dual-layer asymmetric hollow fiber membranes by a simultaneous co-extrusion approach during the past 17 years and conducted a systematic study to investigate the relationship among dual-layer hollow fiber membrane morphology, dope compositions, and spinning conditions. Using Matrimid 5218 as the outer-layer material and polyethersulfone (PES) and its blends as the inner-layer materials, the science and engineering factors to produce dual-layer hollow fiber membranes with high integrity have been investigated and the causes of forming a macrovoid-free structure have been examined. Both the concepts of critical dope viscosity and critical structure–transition thickness have been closely evaluated if they are suitable to explain the formation of a sponge-like structure at certain conditions. Experimental results indicate that the macrovoids in the inner PES dope cannot be easily eliminated by the modification of the inner dope viscosity, but can effectively suppressed by either the addition of polyethylene oxide (PEO) in the PES inner dope or spinning fibers at much higher elongational draw ratios. The effects of different shrinkage percentages of both inner and outer layers on overall membrane morphology were studied and the optimal approach to develop delamination-free interface was proposed.