Saliva is a complex secretion. 93% by volume is secreted by the major salivary glands and the remaining 7% by the minor glands. 99% of saliva is water and the other 1% is composed of organic and inorganic molecules. While the quantity of saliva is important, so is its quality. The components of saliva, its functions in maintaining oral health and the main factors that cause alterations in salivary secretion will be reviewed, the importance of saliva in caries development and bacterial plaque formation will be discussed and its role as an aid to diagnosing certain pathologies will be examined. Variations in salivary flow can be affected, reversibly or irreversibly, by numerous physiological and pathological factors. Saliva plays an essential role in maintaining the integrity of the oral structures, in personal relationships, in the digestion and in controlling oral infection. The part that saliva plays in protecting teeth from caries can be summarised under four aspects: diluting and eliminating sugars and other substances, buffer capacity, balancing demineralisation/remineralisation and antimicrobial action. Saliva is a promising option for diagnosing certain disorders and monitoring the evolution of certain pathologies or the dosage of medicines or drugs. Its advantages as a diagnostic tool include its being easy to obtain and the positive correlation between many parameters in serum and saliva.