For over 200 years, slow sand filtration (SSF) has been an effective means of treating water for the control of microbiological contaminants in both small and large community water supplies. However, such systems lost popularity to rapid sand filters mainly due to smaller land requirements and less sensitivity to water quality variations. SSF is still a particularly attractive process because its operation does not require chemicals or electricity. It can achieve a high level of treatment, which is mainly attributed to naturally-occurring, biochemical processes in the filter. Several microbiologically-mediated purification mechanisms (e. g. predation, scavenging, adsorption and bio-oxidation) have been hypothesised or assumed to occur in the biofilm that forms in the filter but these have not yet been comprehensively verified. Thus, SSFs are operated as 'black boxes' and knowledge gaps pertaining to the underlying ecology and ecophysiology limit the design and optimisation of the technology. The objective of this review is to outline the biological aspects of SSF in to the context of recent developments in molecular microbial ecology.