This chapter has briefly reviewed the development and progression of peripheral-type adenocarcinoma of the lung, focusing particularly on bronchioloalveolar carcinoma consisting of the nonmucus-producing cell type with or without sclerosis. Histoloical examination reveals that scar cancers are rare except in cases of diffuse pulmonary fibrosis and that many nonmucus-producing bronchioloalveolar carcinomas appear to develop from atypical adenomatous hyperplasia, which can be called adenoma or very well-differentiated adenocarcinoma, and to progress stepwise. Stepwise progression in malignancy can be disclosed not only by cytological and histological examination but also by proliferative activity of the tumor, such as mitotic activity, the percentage of DNA-synthesizing cells and the frequency of proliferating cell nuclear antigen-positive cells, the mean nuclear DNA content of tumor cells and occurrence of aneuploid cell lines, and abnormalities of oncogenes (c-Ki-ras, myc family, and c-erbB2), such as point mutation, rearrangement, amplification, and tumor suppressor genes (point mutation and deletion) such as p53.