The term used for all malignant tumours is cancer. Malignant cancer cells are characterized by loss of growth control, invasiveness and metastasis. Benign tumour cells have lost growth control but do not metastasize. Carcinogenesis means induction of cancer and the agents which can induce cancer are called carcinogens. Carcinogens are physical, chemical, biologic and hormonal agents that can cause cancer by damaging or altering DNA. Proto-oncogenes are normal genes which stimulate cell division. Activation of these genes to oncogenes is achieved by mutation. Oncogenes are genes capable of causing cancer. Tumour suppressor genes or anti-oncogenes are normal genes which inhibit cell division. Mutations of these genes cause cancer. Certain enzymes, hormones and antigens are released by tumour cells and used as tumor markers in diagnosis and for monitoring, treatment and recurrence of cancer.