Ichthyoses are a heterogeneous group of cornification disorders characterized by a generalized scaling of the skin. Common types such as ichthyosis vulgaris and X-linked recessive ichthyosis manifest after birth. In contrast, rare congenital ichthyoses are inherited diseases, which at birth typically present collodion membranes or ichthyosiform erythroderma. Syndromic ichthyoses display a variety of outstanding associated non-cutaneous symptoms. Because of their rarity these disorders often pose a diagnostic challenge for the clinician. This review discusses a broad spectrum of 13 isolated types of ichthyoses, 11 different syndromes with associated ichthyosis and four related cornification disorders. The clinical, ultrastructural and biochemical characteristics are described along with the different molecular causes of ichthyosis. Special attention is given to lamellar ichthyosis and non-bullous congenital ichthyosiform erythroderma. The different pathomechanisms causing ichthyosis provide a fascinating insight into the role of various proteins, enzymes, lipids and metabolic pathways involved in terminal epidermal differentiation/keratinisation.