For patients with complaints of either hyposalivation or hypersalivation, or saliva with altered properties, the secretory flow rate can be determined in an easy way to study whether abnormalities are detectable. It is most simple to measure the flow rate of whole saliva. After collection of saliva the oral fluid can be analyzed on rheological properties, such as viscosity and elasticity and also on pH and its chemical composition. Oral fluid is the combined fluid in the oral cavity mainly composed of glandular salivas, and in addition crevicular fluid and serum exsudate. For collection of oral fluid it is of crucial importance to standardize the conditions. The diverse collection methods and their application for patient research with oral complaints are described.