The biology of cancer has been rapidly advancing with new discoveries; therefore, understanding its mechanisms and pathogenesis is an important aspect. This chapter on Cancer Biology brings you an in-depth knowledge about the origin of cancer, its molecular alterations within the host’s cell, and growth. This chapter also illustrates on the ability of cancer to avoid programmed cell death and how it achieves immortality. The aptitude of cancer cells to metastasize and its behavior to expand has also been explicated. With the understanding of biological basis of cancer, there are many innovative therapeutic molecules being developed aside from conventional therapeutics, and various cancer immunotherapies have been illuminated. Finally, we have also discussed about various developments to detect and diagnose the cancer at its metabolic state. The chapter then concludes by elucidating the present challenges and future prospects for cancer biology.