This chapter explores the fundamental principles of molecular biology and genetics that underpin modern radiation oncology practice. It provides key definitions and discusses molecular determinants of radiosensitivity and radioresistance, tumor heterogeneity, and the role of biomarkers in treatment personalization. The chapter also dwells on the emerging concept of systemic effects of radiation therapy and examines the newer technology in the context of molecular oncology. By integrating molecular and genetic insights with clinical radiotherapy, it highlights emerging strategies to optimize therapeutic efficacy while minimizing normal tissue toxicity.