Sterols play important roles in most eukaryotic cells. This chapter demonstrates that inhibitors of sterol biosynthesis have profound biological effects in higher plants and may contribute to unravel and to understand the different roles of plant sterols. Thus far, little is known about sterol functions in higher plants. There are several reasons to explain this lack of knowledge. A major one is that it is still not possible to obtain higher plant cells auxotrophic to sterols similar to yeast and some mammalian cell cultures. Only a few reports on the solubilization and purification of plant sterol biosynthetic enzymes operating downstream to squalene have been published. The extensive purification of other sterol biosynthetic enzymes from plants is undoubtedly a very important goal to be accomplished. The results of mechanistic studies performed with sterol and terpenoid biosynthetic enzymes have shown that several enzymatic reactions involve one or several carbocationic high energy intermediates.