Unlike antibiotics, biocides are multi-targeted antimicrobial agents. Several of the damaging effects reported to occur in the most widely studied organisms, bacteria, may also take place to varying degrees in other organisms. Nevertheless, there is considerable variation in the response of different microorganisms to biocides. Bacteria themselves (Gram-positive and Gram-negative vegetative organisms, mycobacteria and spores) respond differently to biocides and this disparity is widened when yeasts, moulds, protozoa and algae are considered. The underlying reasons for these varied responses are poorly understood at present, but the chemical composition of outer cellular layers is likely to be a factor of prime importance. Other possible contributory factors may be differences in stress responses, the presence of efflux pumps and cells occurring within biofilms or algal mats.