Great progress has been made in the field of hereditary metabolic diseases since the beginning of the past century, when metabolic disorders were not really understood and could only be clinically described. Due to the development of basic sciences and advances in technology, we gained insight in the biochemical and molecular basis of hereditary metabolic diseases. It opened possibilities for their treatment, and also led to the discovery of more metabolic diseases, so today, there are more than 500 inborn errors of metabolism known. Although each of these diseases is quite rare, as a group, however, they affect about 1-2% of newborns and therefore pose a significant health problem. The realization about 50 years ago that some hereditary diseases are curable if timely diagnosed led to the introduction of newborn screening in most countries. Modern technologies in this field allow early diagnosis of more than 30 inborn errors of metabolism. Nevertheless, to diagnose most patients correctly, both selective screening involving teamwork and proper use of current technology are required. In addition to considerable development of diagnostic possibilities, the past decade was marked by advances in the therapy of inborn errors of metabolism. A number of clinical trials are currently underway, promising new and more effective approaches in the treatment of these patients. Thus, the field of inborn errors of metabolism at the beginning of the new millennium continues to be a scientific challenge to modern medicine.