The 2015 WHO Classification of Lung Tumors has undergone significant changes since the publication of 2004 WHO Classification. These changes have resulted from advances in molecular testing and radiology as well as greater use of immunohistochemistry in subtyping tumors. The major changes in the 2015 WHO classification of adenocarcinomas of the lung (resected tumors) are: 1) Discontinuing the terms bronchioloalveolar carcinoma and mixed subtype adenocarcinoma; 2) Adding Adenocarcinoma-in-situ to the list of pre-invasive lesions; 3) Introducing the concept of minimally-invasive adenocarcinoma; 4) Classification of invasive adenocarcinomas based on the predominant subtype; 5) Use of the term “lepidic” for non-invasive component in an invasive adenocarcinoma; 6) Introduction of the term “invasive mucinous adenocarcinoma” for cases previously classified as mucinous bronchioloalveolar carcinoma; 7) Discontinuing the use of clear cell and signet ring cell adenocarcinoma subtypes; 8) Discontinuing the term mucinous cystadenocarcinoma and including them in colloid adenocarcinoma. Reference: Travis WD et al. WHO Classification of Tumours of the Lung, Pleura, Thymus and Heart; 4th edition, pages 26-50. International Agency for Research on Cancer, Lyon, 2015.