Osteogenesis imperfecta (OI) is a group of disorders characterized by fractures with minimal or absent trauma, dentinogenesis imperfecta (DI), and, in adult years, hearing loss. The clinical features of OI represent a continuum ranging from perinatal lethality to individuals with severe skeletal deformities, mobility impairments, and very short stature to nearly asymptomatic individuals with a mild predisposition to fractures, normal stature, and normal lifespan. Fractures can occur in any bone, but are most common in the extremities. DI is characterized by grey or brown teeth that may appear translucent and wear down and break easily. Before the molecular basis of OI was understood, OI was classified into four types on the basis of mode of inheritance, clinical presentation, and radiographic findings. With detailed radiographic and bone morphologic studies and molecular genetic analyses, the classification has expanded to seven types and it is likely that more will emerge. This classification into types of OI is helpful in providing information about prognosis and management, but it should be remembered that many of the types of OI represent an artificial construct on a broad clinical entity. The clinical diagnosis of OI is based on family history, a history of fractures, characteristic physical findings including scleral hue, and radiographic findings. Radiographic findings include fractures of varying ages and stages of healing, wormian bones, "codfish" vertebrae, and osteopenia. Analysis of bone biopsies is an adjunct to the diagnosis of OI type V and OI type VI. Biochemical testing (i.e., analysis of the structure and quantity of type I collagen synthesized in vitro by cultured dermal fibroblasts) detects abnormalities in 98% of individuals with OI type II, about 90% with OI type I, about 84% with OI type IV, and about 84% with OI type III. About 90% of individuals with OI types I, II, III, and IV (but none with OI types V, VI and VII) have an identifiable mutation in either COL1A1 or COL1A2. Such testing is clinically available. Osteogenesis imperfecta types I-V are inherited in an autosomal dominant manner. OI type VII is inherited in an autosomal recessive manner, and the mode of inheritance of OI type VI is not yet certain. For types I-IV, the proportion of cases caused by a de novo mutation in either COL1A1 or COL1A2 varies by the severity of disease. Approximately 60% of individuals with mild OI have de novo mutations; virtually 100% of individuals with lethal (type II) OI or with severe (type III) OI have a de novo mutation. Each child of an individual with a dominantly inherited form of OI has a 50% chance of inheriting the mutation and of developing some manifestations of OI. Prenatal testing in at-risk pregnancies can be performed by analysis of collagen synthesized by fetal cells obtained by chorionic villus sampling (CVS) at about ten to 12 weeks' gestation if an abnormality of collagen has been identified in cultured cells from the proband. Biochemical analysis of collagen from amniocytes is not useful because amniocytes do not produce type I collagen. Prenatal testing in high-risk pregnancies can be performed by molecular genetic testing of COL1A1 and COL1A2 if the mutation has been identified in an affected relative. Prenatal ultrasound examination performed in a center with experience in diagnosing OI, and done at the appropriate gestational age, can be valuable in the prenatal diagnosis of the lethal form and most severe forms of OI prior to 20 weeks' gestation; fetuses affected with milder forms may be detected later in pregnancy when fractures or deformities occur.