This issue of Seminars of Medical Genetics features a series of articles on Smith–Lemli–Opitz syndrome and other disorders of endogenous cholesterol synthesis. Clinically, many of these disorders have been known for decades, but only in the last 20 years have the molecular genetic and enzymatic defects underlying these disorders been delineated. As a group, disorders of cholesterol synthesis are relatively common and contribute significantly to the burden of human disease. Leading experts in their fields present clinical, behavioral, molecular, and therapeutic aspects of these disorders. © 2012 Wiley Periodicals, Inc.