Oral fluid mainly consists of a mixture of glandular salivas. In addition, it is contaminated by some crevicular fluid, containing serum constituents. The contribution of the various salivary glands shows a continuous variation, resulting in wide ranges of concentrations for all constituents of oral fluid. As a consequence, the collection of oral fluid for diagnostic purposes should be standardized. Oral fluid can be used to detect a number of diseases and recent use of illicit drugs. It can also be used to monitor therapeutic drug concentrations. The development of microchips for salivary components offers great possibilities to use oral fluid for point-of-care testing.