Salivary cortisol assay, described for the first time almost forty years ago, has not been expanding until the last decade. Its simplicity, non-invasiveness and the easy repetition of sampling make it an analytical matrix of interest. Since the publication of the recommendations of the American endocrinology society in 2008, salivary cortisol is recognized as one of the three main tests to screen for Cushing's syndrome. In addition, salivary cortisone, the major metabolite of salivary cortisol, still represents a severe potential interferent but could also be a complementary analyte for indications where evaluation of cortisol secretion is sought. Moreover, in the current context of practices and methods harmonization, the problem of lack of standardization presents also for salivary cortisol. This review briefly develops the three main tests of Cushing's syndrome screening to explain the reasons for integrating the saliva test into this screening. Then we will develop the variables that can influence salivary cortisol from a pre-analytic, physiopathological and finally analytical point of view.