The study of plant natural products can be considered to have started in 1806 when Serturner isolated the first of these compounds, morphine, from the opium poppy. Over the next 150 years, numerous elegant and powerful techniques were developed for the isolation and structural identifi­ cation of a variety of classes of these substances. With increasing knowledge, various speculations were put forward on the ways that these compounds were synthesised from pri­ mary metabolites and on their possible physiological, ecolo­ gical and taxonomic importance. These investigations sublimated in the late 1940's with the newly developed application of radioactively labelled precursors to the study vari ous bi osyntheti c pathways and use of rapid chromatagraphic and spectroscopic methods to uncover the structure of a plethora of new compounds and to their wide distribution in various taxa of plants recognize and other organisms. We have now entered a new stage in our investigation of natural products in which several important new problems are being tackled. The present emphasis is on how the various biosynthetic pathways are regulated, the enzymic parameters which determine the chirality of the products, the transfer and modification of components in food chains, and the ways in which a given class of physiologically or ecologically important compounds has been superseded by another during the course of evolution.