A number of possible mechanisms, both physical and chemical, are postulated as being potentially damaging to borehole performance in micaceous aquifers. Laboratory studies have been undertaken to assessthe degree to whichthese mechanisms are operative, and the extent of permeability damage which occurs as a result. Some of the processes studied are unique to micaceous formations, while others are enhanced where mica is present. Two key processes identified by the study are: (i)mineralogical and structural alteration of mica particles as a result of increased groundwater salinity; and (ii) mixing of formation sands, either between layers, or with gravel pack material. The studies together with field evidence show that both of these processes can cause a significant reduction in permeability in the vicinity of a borehole, leading to a deterioration in borehole performance. The degree of deterioration which can result from each of the processes is assessed, and recommendations for operation, maintenance and rehabilitation, to minimize damage are made.