Though the molecular aspect of microbiology has long been recognized, it has greatly expanded in recent years. The molecular study of medical microbiology has revealed conceptual insights and technical approaches that have advanced the subject almost beyond recognition. The biological experiments by Fred Griffiths that identified the pneumococcal transforming principle were the prelude to its identification as DNA, in turn eventually leading to the recognition of genetics as the foundation of molecular microbiology. Similarly, the understanding of DNA at the structural level led to the discovery of the polymerase chain reaction (PCR). The discovery of host-controlled restriction-modification and restriction enzymes was the foundation of genetic manipulation. This molecular approach has provided information about the pathogenesis and prevention of bacterial diseases. In the case of Haemophilus influenzae type b these advances have brought into focus the possible elimination of this virulent childhood pathogen. Since 1995 the whole genomes of an increasing number of bacterial species have been described and this has opened up the omics technologies. These fundamental approaches have opened up new vistas in the diagnosis, treatment and prevention of diseases due to bacteria.