In this chapter, we critically discuss the various mechanisms for CD. Since the kinetics of deposition are intimately related to the mechanism, this aspect is also treated here. This chapter is divided into several sections dealing with different aspects of the mechanisms. However, there is a large degree of overlap of the material in these different sections; indeed, it was often not obvious in which section a specific topic should be placed. While the main discussion of any specific point will be limited to the most relevant section, the same point will often be treated, more briefly, in other sections. However, for those readers who are looking for a specific topic in this chapter, it would be prudent not to be confined to only what appears to be the relevant section. For example, elements of the mechanism, probably with different emphases, are dealt with also in the sections on nucleation and on kinetics, while many issues of nucleation are considered in the mechanistic studies.