This article reviews current concepts in pathologic classification of lung cancer based on the 2004 World Health Organization classification of lung tumors and the 2011 International Association for the Study of Lung Cancer (IASLC)/American Thoracic Society (ATS)/European Respiratory Society (ERS) classification of lung adenocarcinoma. Preinvasive lesions are discussed. The major changes in lung disease diagnosis affected by the IASLC/ATS/ERS classification are presented. For adenocarcinomas diagnosed in small biopsies, specific terminology and diagnostic criteria are proposed along with recommendations for strategic management of tissue and EGFR mutation testing in patients with advanced adenocarcinoma. Histologic criteria are also presented for other tumors.