Metabolic disorder caused by altered levels of metabolism resulting pathophysiological abnormalities often leads to childhood death. Several new developments on metabolic diseases research have been emerging. Gaucher disease is a lysosomal storage disorder caused by glucocerebrosidase gene mutations resulting glucocerebrosidase deficiency. Current studies show that the same gene mutations also contribute to the Parkinson's disease. Tetrahydrobiopterin (BH4) has been widely used in treating patients with Phenylketonuria over a decade. Recent studies reveal that patients treated with BH4 over one-year period showed reduced levels of serum B12, folate and iron intake and therefore patients under chronic BH4 treatment needed to be advised to have additional micronutrients along with BH4. Macrocephaly was used as one of the important features to diagnose Canavan disease. However, a recent study showing a child with aspartoacylase gene mutation developed microcephaly. Hence, Canavan disease phenotype can be either macrocephaly or microcephaly. These are a few examples of recent developments on metabolic diseases research. Therefore, this book was aimed to compose current developments on metabolic diseases research for the use by broad spectrum of experts including Physicians, Neuroscientists, Neurologists, Biomedical researchers, Biochemists, Molecular biologists, Basic Science Researchers and Medical Students.