The disorders of cornification (ichthyoses) comprise acquired and inherited disorders characterized clinically by generalized scaling and histologically by hyperkeratosis. They may arise through defects in the production or maintenance of a normal cornified cell compartment, or both. The stratum corneum is composed of protein-enriched and lipid-depleted corneocytes ("bricks") surrounded by an intercellular domain ("mortar") composed of hydrophobic, lipid-enriched membrane bilayers, and containing desmosomes and a limited array of hydrolytic enzymes. Mechanisms whereby a genetic defect involving either the bricks or the mortar may result in abnormal stratum corneum retention are discussed using ichthyosis vulgaris and recessive X-linked ichthyosis as examples. In addition, epidermal hyperproliferation, which floods the cornified cell compartment with incompletely formed units, results in hyperkeratosis. To date, no primary disorders of epidermal hyperproliferation have been defined. Recent work, however, demonstrates that stratum corneum barrier function regulates epidermal DNA synthesis. For example, in essential fatty acid deficiency, barrier dysfunction is responsible at least in part ror the epidermal hyperproliferation. Defective barrier function due to defective lamellar body secretion may also underlie the phenotypic changes after birth in harlequin ichthyosis; that is, from the massive, constrictive hyperkeratosis of the newborn to an exfoliative erythroderma in survivors. The mechanisms whereby specific defects in cornification result in generalized scaling disease are only beginning to be defined. Yet, even at this early stage, the view of the stratum corneum as a tightly organized structure whose function is highly regulated is emerging. Hence, the disorders of cornification should provide important insights into stratum corneum structure and function.