In recent years, because of huge advances in medicine and technological progress, coupled with the application of increasingly accurate diagnostic tests, the use of saliva for medical purposes has become increasingly important. Saliva is a natural digestive secretion of the major and minor salivary glands that ensures, among other things, the proper functioning of the occlusion system. Other functions include the protection of dental surfaces from various external influences, and involvement in the formation of the food bolus and in the acts of swallowing, digestion, and speech. The composition of saliva is varied. In addition to water, which is found in saliva in far greater quantities than in any other substance, it also contains various substances which are also present in the blood. Their determination in saliva often has a great diagnostic potential in the early detection of both oral and systemic diseases. The diagnosis of selected systemic diseases using saliva is becoming the subject of a growing number of scientific studies. In this paper, the authors have focused on providing the reader with basic information about the physiology of saliva and on presenting the possibilities for its diagnostic application in selected systemic diseases.