Many essential oils are extracted, analysed and their main components are identified, characterised and then published without any biological testing whatsoever. Their useful biological activities can remain unknown for years. Yet, the search for these activities often increases our knowledge of the potential use of oils in therapeutics. Therefore, there is a real need for a simple, reliable and reproducible methods to study the bioactivity of essential oils and their constituents which can detect a broad spectrum of action or specific pharmacological activities in aromatic plants. These methods can then be employed by natural product chemists, pharmacologists and biologists to conduct their scientific research and to valorize natural products. Standardisation of some of these methods is therefore desirable to permit more comprehensive evaluation of plant oils, and greater comparability of the results obtained by different investigators.