The lactonic bitter principles comprise an important group of natural substances. If we exclude from consideration the coumarins, compounds which do not present great difficulties in structural elucidation, then there remains a large family of bitter principles whose biogenesis appears to be of the terpenoid (mevalonic acid) type. Compounds of this latter kind have presented challenging problems for generations, but it is only in the last decade that their constitutions have received serious attention. Because of the limited time available for this lecture, only work from our own group can be discussed. This does not, of course, mean that others have not also been making important contributions to the field. We may commence by listing some of the interesting constitutions that our group has studied during the last few years. First comes tenulin (I) 1, chiefly remarkable for the ease with which it rearranges under very mildly alkaline conditions (boiling London tap water!) into isotenulin (II), which has a true acetate grouping. Secondly, we may mention lactucin (III) 2, a compound of alleged medicinal value, which has an interesting cross-conjugated dienone system. Thirdly, comes geigerin (IV) 3, a compound to which we shall make further reference later. Fourthly, let us consider pyrethrosin (V) 4, of especial interest as it was the first sesquiterpenoid in which a ten-membered ring was shown to be present. This feature was recognized as being of biogenetic significance, a view which has been abundantly confirmed by later work'', These compounds contain sesquiterpenoid skeletons based directly upon a cyclized farnesol chain without rearrangement of the carbon skeleton. The diterpenoid derivative, columbin (VI), the main bitter principle of Colombo root, is different in that the two methyl groups have, according to biogenetic hypothesis, been moved from their original positions in a skeleton as in (VII)? These examples will serve to show the wide variety of functional groups that one finds in the lac tonic bitter principles. The problems of structural elucidation are, in fact, as difficult as one wishes to make them. Perhaps we can illustrate this, as well as the interesting types of reaction that can be discovered incidentally to the investigations, by considering our recent work on the citrus bitter principles. Limonin is the characteristic bitter principle of citrus species occurring in all parts of the plant. The literature on this compound has been detailed in a recent paper, and we shall cite here only those references especially pertinent to our own work. Our investigations on the constitution oflimonin were initiated in Glasgow in 1956, and have been continued to the present time. From the outset, we