Tumorigenesis is a multi-step process in which exogenous and endogenous mutational events lead to accumulation of DNA mutations resulting in loss of cellular growth control. During the last decades knowledge about the molecular mechanisms of DNA mutations and the cellular systems that counteract the degeneration of the genome has increased rapidly. This expansion was driven in part by clinical and molecular investigations of syndromes with genetic in-stability. These syndromes are typically associated with an increased predisposition for tumor formation. They include nucleotide excision repair defective syndromes, mismatch repair defective syndromes, and the chromosome break-age syndromes. These diseases mirror vividly the phenotypic consequences of the failure of important cellular systems for the maintenance of the genomic integrity. They significantly contribute to our understanding of the molecular mechanisms of (skin) carcinogenesis and for the development of preventive and therapeutic anticancer strategies.