This chapter is a primer on oncology in general. It presents a description of the basics of cell biology and cell division, including the regulators of the cell cycle and the cell signaling pathways, particularly those involved in ophthalmic tumors. This chapter also explains the mechanisms of tumorigenesis, including the roles of tumor genetics, epigenetics, environmental carcinogens, and hereditary cancer predisposition. It also details the hallmarks of cancer, including those contributing to tumor initiation, promotion, and progression. The different aspects of tumor pathology are highlighted, including the histological and cytological features of malignancy, the various standard histopathology diagnostic methods, tumor cytopathology, tumor cytogenetics, and tumor molecular genomics. Tumor grading and staging systems are highlighted.