Electron microscopy (EM) is still an important tool for the investigation of infectious diseases, despite the introduction of powerful new methods, mainly involving the polymerase chain reaction. Particularly in the field of parasitic protozoology associated with AIDS, where many new species of human pathogens have been recognized in tissue biopsies, EM remains an essential 'catch-all' diagnostic method. The resolved ultrastructural details of these newly recognized parasites allows a unique insight into the biology of these organisms. The information produced by EM is different, but complementary, to that provided by alternative methods.