Transesterification is the technological route more used for biodiesel production, and can be applied on a small scale, as in laboratories, or in industry, producing millions of gallons of biofuel. Although the esterification also results in biodiesel and is recommended when the raw material is composed of oils rich in free fatty acids, this technique is applied commercially in few industries. That’s why we decide to mention in this chapter only the transesterification process. In this section, mechanisms of the transesterification reaction are going to be explained, identifying all process variables affecting the biodiesel yield and finally some important optimization studies will be presented.