Tumorigenesis is a long, multistep evolutionary process driven by somatic mutations, clonal selection, microenvironmental pressures, and genomic instability. Most cancers grow unnoticed for years, accumulating driver mutations that confer proliferative, survival, and invasive advantages. As tumors enlarge, subclonal diversity increases, and the likelihood of metastatic dissemination rises. Metastasis accounts for over 90% of cancer-related deaths, and even small primary tumors may shed malignant cells before clinical detection. Tumor evolution is shaped by both intrinsic mechanisms, such as replication errors, defective DNA repair, and oxidative stress, and extrinsic carcinogenic exposures including tobacco smoke, UV radiation, alcohol, and chronic inflammation. Normal cells rely on highly efficient DNA repair pathways, but defects in caretaker genes or microenvironmental stresses like hypoxia generate a mutator phenotype, accelerating malignant progression. Although tumors can harbor thousands of mutations, only a minority function as drivers; most are passengers. Across cancer types, diverse genetic alterations ultimately converge on a limited set of pathways underlying the hallmarks of cancer. Emerging concepts highlight that tumor evolution is not purely genetic. Epigenomic heterogeneity and microenvironmental constraints also shape malignant behavior, enabling adaptation and therapy resistance. Together, these processes explain the individuality of cancers and emphasize the importance of prevention, early detection, and personalized therapy.