Classifcation of lung adenocarcinoma was largely revised in the 4th edition of WHO classifcation of tumors of the lung, pleura, thymus, and heart published in 2015. This chapter deals with the major changes in the adenocarcinoma classifcation, briefly describing the defnition, gross and histopathological fndings, genetic profles and clinical features of each subtype, and variants of lung adenocarcinoma. Special reference was also made to the prognostic aspects. The new concepts of adenocarcinoma in situ and minimally invasive adenocarcinoma are especially important from the prognostic point of view because of their virtual connotation as 100% curable cancers if resected completely. Each subtype of invasive adenocarcinoma may be categorized into either good, intermediate, or poor prognostic group. Much progress has been made regarding the genetic profles as well, such as the occurrence of EGFR and KRAS mutations, ALK fusion genes and recently discovered alterations, and NRG1 fusion genes in association with adenocarcinomas with certain characteristics. A brief overview of the major changes in the lung adenocarcinoma classifcation in this chapter will help physicians, radiologists, and pathologists grasp the signifcance and meaning of the histopathological diagnosis according to the new WHO classifcation.