This lecture covers the genetic and epigenetic changes in cancer cells, the control of the cell cycle, proto-oncogenes, tumor suppressor genes, familial forms of cancer, and the impact of carcinogens and infectious agents on cancer development. It explains the fundamental properties of cancer, the proof of cancer cell clonality, the multistep process of cancer development, and the role of driver mutations, genomic instability, and epigenetic modifications in tumors.