A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues and persists in the same excessive manner after cessation of the stimuli, which evoked the change. The neoplasia tumors are classified as benign and malignant, depending on the biological behavior of a tumor. Benign tumors have a relatively innocent microscopic and gross characteristic. Benign tumors remain localized without invasion or metastasis. Cancer is the general term used for malignant tumors. Malignant tumors invade or infiltrate into the adjacent tissues or structures. The main highlights of this chapter are—nomenclature of neoplasms, carcinoma in situ, metastasis, events in invasion and metastasis, precancerous conditions/lesions, molecular basis of cancer, hallmarks of cancer, etiology of cancer (carcinogenic agents), laboratory diagnosis of cancer, common specific epithelial tumors, mesenchymal tumors, peripheral nerve sheath tumors, and skeletal muscle tumors.