Air-to-air membrane enthalpy exchangers (MEEs) are being considered as key components in the new generation of heating, ventilation and air conditioning (HVAC) systems. MEEs are an environmentally friendly solution to reduce building energy consumption by preconditioning the incoming air using the exhaust air. Semi-permeable membranes are used in MEEs to separate the air streams and recover both sensible and latent heat from one air stream to the other. This review addresses membrane characteristics, performance improvement techniques, design optimisation efforts and the effects of the operating conditions on the performance of MEEs. It was found that membrane properties can be classified into important properties and less important ones. Polymeric membranes are widely used in MEEs. The hybrid flow configuration could efficiently overcome the manufacturing difficulties of pure counter flow configuration while maintaining the high performance of MEEs. Various techniques have been applied to improve the heat and mass transfer performance of MEEs. However, some of them increased pressure drop. Although design optimisation could improve the performance of MEEs, limited attention has been focused on this topic. This study could provide a better understanding of the research activities and trends of MEE technologies and guide practical applications of MEEs for energy savings.