Cancer remains one of the most complex challenges in modern medicine, demanding a solid understanding of its biological foundations. This textbook offers a clear and structured approach to cancer development, progression, and therapeutic response, effectively bridging molecular mechanisms with clinical practice. Readers gain in-depth insight into oncogenes and tumor suppressors, multistep tumorigenesis, the tumor microenvironment, metastasis, cancer immunology, and the rapidly expanding field of targeted and precision therapies. Designed with a strong didactic concept, the book features informative boxed sections highlighting key concepts, numerous high-quality illustrations, and a comprehensive glossary of technical terms. Its scaffolded structure guides learners from fundamental principles to advanced applications, making it particularly well suited for students. Based on Prof. Carsten Carlberg’s long-standing lecture course “Cancer Biology,” taught for many years at the University of Eastern Finland, and enriched by Dr. Eunike Velleuer’s clinical expertise, the book combines academic rigor with clinical relevance. The completely revised and expanded second edition includes new content on DNA methylation–based tumor classification, liquid biopsies, cancer epigenomics, aging as a cancer risk factor, sex differences, and cancer survivorship. Cancer Biology: How Science Works is an essential resource for students, practitioners, and anyone seeking a clear and scientifically grounded understanding of cancer and its clinical implications.