Twenty years after the idea of this Carotenoids book series was first discussed, we are finally reaching the end of the project. Carotenoids Volumes 4 and 5 now move us into the great field of biology, and cover the functions of carotenoids and the actions of carotenoids in nutrition and health. In the classic 1971 Isler book Carotenoids, carotenoid functions were covered in just one chapter. Now, thanks to technical developments and multidisciplinary approaches that make it possible to study functional processes in great detail, this subject is the most rapidly expanding area of carotenoid research, and occupies two full volumes. These can be used as stand-alone books, but they are really planned to be used together and to complete the coverage of the carotenoid field begun in earlier volumes, as the final part of a coordinated series. The general philosophy and strategy of the series, to have expert authors review, analyse and present information and give guidance on practical strategies and procedures is maintained in Volumes 4 and 5, though the subject matter does not lend itself to the kind of detailed Worked Examples that were featured in earlier volumes. It has also been the aim that these publications should be useful for both experienced carotenoid researchers and newcomers to the field.