Surfactants, both chemical and biological, are amphiphilic compounds which can reduce surface and interfacial tensions by accumulating at the interface of immiscible fluids and increase the solubility, mobility, bioavailability and subsequent biodegradation of hydrophobic or insoluble organic compounds. Investigations on their impacts on microbial activity have generally been limited in scope to the most common and best characterized surfactants. Recently a number of new biosurfactants have been described and accelerated advances in molecular and cellular biology are expected to expand our insights into the diversity of structures and applications of biosurfactants. Biosurfactants play an essential natural role in the swarming motility of microorganisms and participate in cellular physiological processes of signaling and differentiation as well as in biofilm formation. Biosurfactants also exhibit natural physiological roles in increasing bioavailability of hydrophobic molecules and can complex with heavy metals, and some also possess antimicrobial activity. Chemical- and indeed bio-surfactants may also be added exogenously to microbial systems to influence behaviour and/or activity, mimicking the latter effects of biosurfactants. They have been exploited in this way, for example as antimicrobial agents in disease control and to improve degradation of chemical contaminants. Chemical surfactants can interact with microbial proteins and can be manipulated to modify enzyme conformation in a manner that alters enzyme activity, stability and/or specificity. Both chemical- and bio-surfactants are potentially toxic to specific microbes and may be exploited as antimicrobial agents against plant, animal and human microbial pathogens. Because of the widespread use of chemical surfactants, their potential impacts on microbial communities in the environment are receiving considerable attention.