Copper is an essential component of a number of important enzymes. Efficient systems have developed for providing sufficient copper for essential functions, while eliminating excess to avoid tissue toxicity. Copper transport is disrupted in two human diseases: Wilson disease and Menkes disease. Both have defects in copper transporting membrane proteins. Many other proteins are involved in copper transport. Some of these proteins have been identified through a study of the similar copper pathway in yeast. This suggests other copper transport diseases are yet to be discovered. Molecular diagnosis holds promise for reliable diagnosis of patients. Testing of flanking markers is a reliable way to detect presymptomatic sibs of a definite patient.