Cancer is a class of diseases in which a group of cells display the traits of uncontrolled growth, invasion, and sometimes metastasis. These three malignant properties of cancers differentiate them from benign tumors, which are self-limited, do not invade or metastasize. Most cancers form a tumor but not all cancer e.g. leukemia.Cancer may affect people at all ages, even fetuses, but risk for the more common varieties tends to increase with age. Nearly all cancers are caused by abnormalities in the genetic material of the transformed cells. These abnormalities may be due to the effects of carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents. Other cancer-promoting genetic abnormalities may be randomly acquired through errors in DNA replication or are inherited and thus present in all cells from birth. Complex interactions between carcinogens and the host genome can explain mechanism of cancers develop after exposure to a known carcinogen.This book addresses the biomolecular mechanisms of new aspects of genetics in the initiation of Cancer and progression of Tumor.