Iron bacteria are well known to borehole operators as the cause of iron biofouling—the build-up of orange-coloured slimes and encrustations on casing, pump and pipe surfaces. In addition to biofouling, these nuisance organisms play a part in: enhancing the corrosion of borehole, pump and pipe components; influencing the chemistry of polluting acidic mine drainage waters; and promoting the accumulation of soluble iron in groundwaters. This paper highlights the diversity of organisms which may be classed as iron bacteria, briefly describes the behaviour of iron bacteria in boreholes and aquifers and outlines the deleterious effects that their activity can have on the efficient functioning of groundwater abstraction systems. However, it would be wrong to classify iron bacteria only as a nuisance. There is potential for harnessing the activities of these organisms in a positive way and particular reference is made here to the benefits of biological removal of iron and arsenic from groundwaters.