Saliva is a secretion of the salivary and mucous glands and is of major importance in the maintainance of oral health. Over the last few decades, saliva has been evaluated as a diagnostic fluid in medicine for determining systemic disease markers as well as for monitoring numerous drugs, narcotics, and hormones. The biochemical analysis of saliva is particularly important in dentistry. The estimation of the risk of appearance and diagnosis of disease, monitoring of disease progression, evaluation of therapy efficacy for caries, periodontitis, premalignant and malignant oral lesions, as well as infectious diseases of the oral cavity, can be assessed by analysing different constituents of saliva. Individuals at risk of caries can be identified using tests that determine saliva flow rate, saliva buffer capacity, and colonisation of the oral cavity by cariogenic bacteria. Today, these rapid and simple diagnostic tests are used routinely in caries risk determination. The study and use of saliva-based diagnostics have increased over the last few decades. Clinical testing of saliva shows much promise. However, there is a need for much additional research in this area, before the true clinical value of saliva as a diagnostic fluid in dentistry can be determined.