The purpose of this chapter is to give sufficient background in the chemical principles involved in CD. For those with a good background in chemistry, a quick read through this chapter as a refresher course will probably be sufficient. However, the chapter is written, to a large extent, keeping in mind that not all readers will have a good chemistry background. The emphasis is on a qualitative or semiquantitative understanding of the principles involved, sufficient to understand the concepts that arise throughout the book. A deliberate policy has been made not to go too deeply into the details of these fundamentals where it is considered unnecessary; references to further reading will be given for those who wish such additional detail.