Studying the vaginal microflora is not only fascinating, with many discoveries to be made, it is also a very practical way to help women get rid of bothersome and sometimes dangerous infections. Gram-stained vaginal preparations, Pap smears, specific cultures, and nucleic acid detection techniques can be used to diagnose the constituents of the vaginal flora, but in trained hands office-based microscopy of a fresh vaginal smear, preferably using a x400 magnification phase-contrast microscope, allows almost every diagnosis and combination of diagnoses imaginable. In this chapter I will address the pros and cons of the tools that are in use to study vaginal flora, and discuss the different types of bacterial flora and the difficulties encountered in reaching the correct diagnosis of pathological conditions. The 'intermediate flora' is addressed separately, and a new entity--'aerobic vaginitis'--is discussed. Future research should focus on the interaction between infecting microorganisms and host defence mechanisms, as both together generate the pathogenicity of these conditions.