Recent efforts in the field of used water treatment and water reclamation have led to the development of a number of innovative high retention membrane bioreactor (HRMBR) systems. These systems invariably combine a high rejection membrane separation with a biological treatment. A common positive outcome of these systems is that smaller size organic contaminants are effectively retained, which facilitates their biodegradation and thus produces high quality product water. This provides the desired high level of separation, but also leads to salt accumulation with potentially adverse effects on the operations. The effects of elevated salt condition are complex, and impact on aspects covering physicochemical parameters, microbiology and membrane performance. The salt concentration factor is an important operating parameter to be optimised in the HRMBR systems. This paper aims to elucidate the important issues associated with the use of HRMBR systems under elevated salt conditions up to 50gL(-1).