Hereditary or inherited ichthyoses are monogenic disorders of cornification due to mutations in genes involved in the skin barrier function. Skin changes involve most of the tegument, and are characterised by scales of various forms and severities. They are often associated with erythema. The classification should be clinically based, and distinguishes syndromic from nonsyndromic ichthyoses. The diagnosis of the form of ichthyosis is usually very difficult.