Genetic disorders of collagen comprise a heterogeneous group of disease with pleiotropic manifestations and monogenic inheritance. They are caused by mutations in genes encoding collagen proteins or enzymes involved in collagen biosynthesis. New insights into the molecular basis of two of these conditions are presented here. In Osteogenesis Imperfecta (OI) characterized by bone fragility, mutations in the COL1A1 or the COL1A2 genes encoding type I collagen are associated with a wide spectrum of phenotypes, varying from mild to severe and lethal conditions. The mild forms are usually caused by haploinsufficiency mutations in the COL1A1 gene while the severe and lethal forms result from dominant negative mutations in COL1A1 or COL1A2. Several collagen types are involved in the Ehlers-Danlos syndromes (EDS), of which skin hyperelasticity, joint hypermobility and vascular fragility are the principal features. Six different genetic subtypes are recognised. Structural mutations in collagen type I, III or V underly the dominant forms, whereas recessive EDS subtypes are associated with enzymatic defects of collagen type I biosynthesis. Clinical application of biochemical and molecular collagen analysis has greatly improved quality of diagnosis, genetic counselling and management of these heritable disorders.