This book summarizes the substantial work that has been accomplished in the treatment of inborn errors of metabolism with simple molecules. These agents are discrete, often of natural origin, and provide predictable therapeutic responses. As such, they avoid many of the practical difficulties associated with gene and protein therapies. This handbook will enable interested clinician scientists and others to rapidly survey the field, thus ascertaining what has been done and as well as future directions for therapeutic research. Its important introductory chapters discuss the infrastructure of the field. These chapters focus on an introduction to pharmacokinetics and pharmacodynamics, a description of the FDA Office of Orphan Products, and a summary of the operation of the National Institutes of Health Office of Rare Diseases. The remainder of the book is devoted to a review of small molecule therapy for genetic diseases. The book closely analyzes the cofactors used to augment the function of defective enzymes and the compounds that are able to utilize an alternative pathway in order to avoid the consequences of the metabolic block present in the patient. Among other therapies, the authors discuss the use of zinc and tetrathiomolybdate to treat Wilson's disease and the use of cysteamine to treat nephropathic cystinosis.