A short overview on carcinogenesis and epidemiology is given, as far as it is necessary to understand the resulting morphology. Molecular biology and genetics are discussed in the chapters on Molecular Pathology and Metastasis. Here we have to focus on mechanisms of carcinogenesis to that moment an in situ carcinoma has been formed and the primary tumor is established. Not much knowledge is available on the genetic mechanisms of benign epithelial tumors, whereas some data are available for mesenchymal tumors. This chapter is organized starting with benign epithelial tumors, followed by malignant epithelial tumors, which are the most common lung tumors, and benign and malignant mesenchymal tumor. Also, rare tumors such as germ cell tumors, lymphoreticular tumors, and finally childhood tumors are discussed. A focus will also be on in situ lesions.