Molecular toxicology is a field which deals with the study of molecular mechanism of chemicals which causes toxicity and effects of various chemical components on living organisms. This form of toxicology examines both naturally occurring and synthetic chemicals are the effects of genetic, physiologic and environmental factors on organisms are also considered. This field is very interesting because the fundamental aspects of biology can be revealed by mechanisms of toxicity. For e.g. toxin such as paraquot or DCMU inhibit the process of photosynthesis. Toxin such as rotenone and cyanide interfered with the electron transport chain .Therefore it reveal the existence of the Krebs cycle, a central component of cellular energy metabolism. Understanding of mechanisms of toxicity is essential for meaningful extrapolation of results obtained in experimental systems, for the risk assessment of human exposure to chemicals.