In the last 30 years, homocysteine has been the subject of medical and biochemical research as an indicator. Its concentration in body fluids indicates the hereditary disorder, homocystenuria, and arteriosclerosis and cardiovascular diseases in general. On the basis of improved methods of homocysteine determination in blood, different levels of homocysteinemia can now be distinguished indicating different kinds of diseases which require different medical treatment. In the newly outlined metabolic reaction schemes in this paper, homocysteine appears as an intermediate in several vitally important interconnected cycles. Under normal physiological conditions, its plasma concentration remains within standard limits, while its increase signals dysfunction of some of the essential pathways affecting membrane transport, redox and pH equilibria and energy production in the organism. In these concerted metabolic processes S-adenosylmethionine, folates and vitamins B6 and B12 as well as NO play important roles.