Since the early phase of the Stone Age, different plants and their derivatives have been used to relieve pain and cure diseases. It is estimated that about 80% of some Asian countries population use herbal medicine. Herbal distillates are aqueous products extracted from various plant tissues, which are used in traditional medicine area in some countries. Due to many important physiological roles of methanol in land plants' life, different concentrations of it normally exist in all plants' products, including herbal distillates. The lack of necessary standards and proper regulatory methods has led to a severe increase of methanol in these products. Methanol is potentially toxic for the human body owing to its metabolites (formaldehyde and formic acid). Recent reports have indicated that constant drinking of some kinds of these products contaminated with nearly high amounts of methanol during a long time could create the chronic type of methanol poisoning which its only symptom has been blurred vision, leading to nervous blindness. This has caused many problems and concerns in the involved countries. Different aspects of this problem are briefly investigated in this book.