Cellular transformation and carcinogenesis are considered a multistep process starting with DNA lesions and progressing through loss of cellular growth control and increase invasive and metastatic properties. This book focuses on the demonstration of important steps in carcinogenesis, ranging from chemical interaction of carcinogens with cellular DNA in experimental tumors and cell lines to the analysis of selected human tumors. Special importance is given to the clarification of mechanisms of DNA lesions caused by chemical carcinogens and UV irradiation. Leading scientists describe in review articles the actual state of the art in their specific fields. The reader is informed of trends and scientific approaches to the recognition of pathogenetic mechanisms and factors in malignant tumors.