An important part of quality control of herbal drug preparations is the evaluation of the chemical stability of a finished product during the storage period. Measuring chemical stability is very challenging due to the complexity of a plant extract, which may contain thousands of different compounds. Moreover, the presence of enzymes like glycosidases, esterases or oxidases plays an important role in the breakdown of secondary plant metabolites. This is of particular significance for liquid herbal products. Many reports so far have focused on the stability of an isolated secondary metabolite and its decomposition products. However, results from these data do not always accurately reflect the chemical stability of the compound in an extract. Other metabolites, the so-called concomitant compounds, present in the extract, may influence the overall stability. This has been shown for extracts containing organic acids like citric acid or malic acid, which enhance the stability of phenolic compounds present in the same product. The current paper gives an overview of the parameters influencing the stability of herbal extracts, describes analytical approaches to measure chemical stability of dry and liquid plant extracts, and gives a summary of the stability data available on extracts of some of the important medicinal plants.