Phytosterols are synthesized via the mevalonate pathway of terpenoid formation and arise from the initial cyclization of 3S-squalene-2,3-epoxide. Plant steroids are derived from sterols and comprise steroid saponins, steroid alkaloids, pregnanes, androstanes, estranes, ecdysteroids, withanolides and cardiac glycosides. The typical route of sterol and steroid biosynthesis follows the cycloartenol pathway, whereas the lanosterol route seems to be operative mainly in fungi and animals. It was demonstrated, however, that both sterol pathways can be operative in higher plants. Crucial steps in the conversion of cycloartenol/lanosterol to sterols are the events leading to the removal of the methyl groups at C-4 and C-14. Meanwhile, all steps in the sterol pathway have been elucidated and the respective enzymes/genes characterized. The biosynthetic pathway leading from phytosterol precursors to the cardiac glycosides – important compounds in the treatment of cardiac insufficiency in humans – was basically deduced from studies using radiolabelled precursors. The more recent identification and characterization of several enzymes/genes involved in pregnane and cardenolide metabolism, such as 3β-hydroxysteroid dehydrogenase and progesterone 5β-reductase, have further clarified the pathway. Brassinosteroids (BRs) are hydroxylated derivatives of cholestane and they are specific plant steroid hormones that are essential for normal plant development. The biosynthesis of BRs has mainly been studied in Arabidopsis thaliana. Many of the genes encoding biosynthetic enzymes have been cloned using mutants of Arabidopsis thaliana, pea, tomato and rice which revert to a wild-type phenotype following treatment with exogenous BRs. Phytoecdysteroids are related in structure to the invertebrate steroid hormones. Their biological significance in plants is still under discussion. The understanding of the biosynthetic pathway(s) for phytoecdysteroids is very limited. Steroid saponins constitute a vast group of glycosides present almost exclusively in the monocotyledonous angiosperms, and occurring in only a few dicotyledonous families. As far as enzymatic and genetic aspects are concerned, the biosynthesis of steroid saponins (including the steroid alkaloids) has not been studied extensively. The withanolides are C₂₈-steroids and biogenetically related to the steroid saponins in that they are derived from ergostane-type sterols. These compounds appear to be specific for the Solanaceae and their biosynthesis has not yet been studied at the enzyme/gene level.