At a time when much money has been invested in scientific research in relation to detection, surveillance and protection against bioterrorist attack, the world is continuing to be under attack from nature's own biological weapons - pathogens, and of particular significance are those that are deemed to be emerging. The aim of this review is to address the topic of emerging pathogens in medical microbiology by: (i) defining what is meant by an 'emerging/re-emerging' pathogen; (ii) highlighting the reasons that have attributed to the incidence of emerging/re-emerging pathogens; (iii) providing an overview of the different groups of emerging pathogens; and (iv) highlighting different patient and community groups in which various pathogens, which are not novel, have emerged and that are of medical significance.