A surway is presented on the authors' work in the field of volatile oil research. The gas chromatographic method previously used for analysis of essential oils was transformed to capillary gas chromatographic conditions. The method is also suitable for separation of compound-pairs frequently occurring in essential oils (peppermint, rosemary, lavender, sage, clary sage, thyme oils). Beside the gas chromatographic analysis of essential oils, which was necessary for their standardization and qualification, the influence of different extraction methods and some biological facts e.g. the ontogenesis on the change of essential oil composition are also discussed. It has been established that the water steam distillation from acidic medium can be more advantageous than the traditional one, if the volatile terpene derivatives were bound in form of glycosides or dimeric quajazolide lactons were present in plant (oregano, Sideritis, wormwood oils). Comparing the composition of essential oil obtained by water steam distillation and supercritical fluid extraction (SFE) it was found that the SFE fractions are richer in ester constituents because the possibility of hydrolysis is reduced, and the oils are more valuable than the classic oils. On the other hand, when the transformation processes are important (chamomile), the distillation is the better method. The change of essential oil composition of Ocimum basilicum L. and Anthriscus cerefolium L. was also studied during the vegetation period. It has been established that in budding and early flowering stages the basil oil was rich in monoterpenes; the quantity of sesquiterpenes and phenylpropane derivatives increased only in later stadiums. Finally the extraction and analytical processes are discussed which are used for standardization of complex plant preparations which contained essential oil as active agent and represented various medicinal forms.