Aldehyde dehydrogenase (ALDH) is a ubiquitous enzyme located in nearly all mammalian tissues. It catalyzes the irreversible oxidation of a range of aliphatic and aromatic aldehydes to their corresponding carboxylic acids. There are multiple isoforms of ALDH which are subdivided into three classes. Class I includes the cytosolic isoforms. Class II includes the mitochondrial isoforms. Class III includes the microsomal, cytosolic tumor specific, and cytosolic dioxin-inducible forms. At least twelve human ALDH isoforms have been identified. Mutations of many of these proteins such as ALDH1, ALDH2, ALDH4, and ALDH10 have been implicated in multiple human metabolic disorders and clinical abnormalities. At the amino acid level, human ALDH isoforms exhibit a wide range of diversity (15% to about 80%). However, multiple protein regions have been highly conserved and are important for functional activities. A well-characterized member of the human ALDH family is ALDH1. It plays a major role in the biosynthesis of retinoic acid from retinol (vitamin A). Retinoic acid, the biologically active form of retinol, is a regulator of cellular proliferation, differentiation, and survival.