Biotransformation is a chemical reaction catalyzed by cells, organs or enzymes. It is defined as a process by which functional groups of organic compounds are modified by living cells to a chemically different product. Biotransformation explores the unique properties of biocatalysts, namely their stereo and region specificity and their ability to carry out reactions at extreme temperatures and pH values. Biotransformation can be used to carry out specific conversions of complex substrates using plant, animal or microbial cells or purified enzymes as a catalyst. Biotransformation is different from biosynthesis where complex products are assembled from simple substrates by entire cells, organs or organisms. They are also different from biodegradations in which complex substances decompose to simple. Biotransformation has great potential to generate new products or to produce known products more efficiently. The production of food metabolites, fine chemicals and pharmaceuticals can be achieved by biotransformation using biological catalysts. Cell suspension cultures, immobilized cells, hairy root cultures may be useful for the production of food additives and pharmaceuticals by biotransformation process. Plant cells for biotransformation are selected for two main reasons. Plant cells are often able to catalyze stereospecific reactions, resulting in chirally pure products. They can carry out regiospecific modifications that are not carried out easily by chemical synthesis or microorganisms. These reactions include reduction, oxidation, hydroxylation, acetylation, esterification, glycosylation, isomerization, methylation, demethylation, epoxidation, etc.