Reaction mechanisms must be considered if sol-gel chemistry is to be developed as anything other than an empirical art. In this chapter we shall describe several mechanistic principles which allow some prediction of the effects of varying the nature and amounts of the silicon precursor, co-solvent and water, and of variables such as pH and temperature. For convenience, the stages of sol-gel processing described in chapter 1 will be considered separately, although it must be realised that this is an artificial scenario since in real systems several of the steps may occur concurrently.