Molecular biological techniques enable rapid detection of pathogenic micro-organisms in water by providing levels of sensitivity and specificity difficult to accomplish with traditional culture-based assays which take days to achieve. Our understanding of the composition, phylogeny, physiology and functional of microbial communities in the environment have been updated by molecular techniques. This book describes the direct detection of pathogens by molecular biologic techniques (ie: techniques based on the analysis of the nucleic acid content (DNA and RNA) of pathogens).