Saliva is secreted from the salivary glands and has multiple functions, including mouth cleaning and protection, antibacterial effects and digestion. With the rapid advancement in salivaomics, saliva is well recognized as a pool of biological markers. Saliva, as a non-invasive and safe source, could be a substitute for blood in the diagnosis and prognosis of diseases. This review summarizes the latest advancements in saliva-related studies and addresses the potential value of saliva in the early diagnosis of oral diseases, such as dental caries and periodontal disease, as well as cancer, diabetes and other systemic disorders. Recently with the outbreak of COVID- 19 in world, saliva again gained its importance because it is capable of diagnosing cases in early stages. The present review article highlights the diagnostic value of saliva in early detection of COVID- 19 infection. Key words: COVID, diagnosis, saliva.