In the new classification criteria for lung adenocarcinoma, there are some noteworthy changes concerning pathology: for the first time, classification methods applicable to surgical resection specimens, small biopsies and cytology, respectively, are proposed; the concepts are updated and changed considerably, such as the names of bronchoalveolar carcinoma (BAC) and mixed adenocarcinoma are no longer used, but replaced by the names of adenocarcinoma in situ (AIS) and microinfiltrative adenocarcinoma (MIA); the A comprehensive and detailed histological diagnostic model is advocated for invasive adenocarcinoma, etc. The main differences between the new classification criteria and the 2004 World Health Organization (WHO) lung tumor classification criteria are summarized. The new classification method was proposed for the first time for surgical resection specimens, small biopsies and cytology, respectively. Approximately 70% of lung cancers are diagnosed on the basis of small biopsies and cytology, so the new classification provides more detailed guidelines for small biopsies and cytology specimens. If the pathologist is unable to classify the tumor definitively on the basis of light microscopy, further classification should be done with the help of immunohistochemistry and/or histochemical staining, etc., with a note in the pathology report that the classification was derived on the basis of immunohistochemistry or histochemical staining performed. The term histologically indeterminate type NSCLC (NSCLC-NOS) should be used as sparingly as possible. In addition, the new classification criteria have specific requirements for specimens. The diagnosis of AIS, MIA, or large cell carcinoma should not be made in small biopsies or cytology specimens. This is because such diagnoses must be made on the basis of a comprehensive sampling of the tumor. The new classification also suggests that a coordinated approach to biopsy tissue specimens is essential, especially for small biopsies and cytology specimens, and that as much high quality tissue as possible should be available for molecular studies. First, the new classification recommends that the names BAC and mixed adenocarcinoma should no longer be used, but be replaced by AIS and MIA. AIS is defined as a small adenocarcinoma (≤3 cm) with limited, squamous growth along the alveolar wall without interstitial, vascular or pleural infiltration. AIS and MIA usually present as non-mucinous or very rare mucinous subtypes, and both types of patients have 100% or near 100% disease-specific survival rates, respectively, if they undergo radical surgery. Second, invasive adenocarcinoma can be classified into subtypes with predominantly squamous, follicular, papillary, and solid growth patterns, with a new subtype of “micropapillary growth pattern” recommended because of its association with poor prognosis. The former WHO classification of clear cell adenocarcinoma and indolent cell adenocarcinoma were included in the solid-based subtypes. Secondly, the variants of invasive adenocarcinoma include invasive mucinous adenocarcinoma (formerly mucinous BAC), colloid adenocarcinoma, fetal adenocarcinoma, and intestinal adenocarcinoma, and the elimination of mucinous cystic adenocarcinoma from the former WHO classification, which is considered as a local morphological manifestation of colloid adenocarcinoma. The intestinal type, on the other hand, is a newly proposed subtype, which should be morphologically differentiated from adenocarcinoma of gastrointestinal origin. Finally, a comprehensive and detailed histological diagnostic model is advocated for invasive adenocarcinoma, instead of generalizing it to a mixed subtype. An example of diagnostic pattern: lung adenocarcinoma with a predominantly solid growth pattern, 10% with an alveolar-like growth pattern and 5% with a papillary growth pattern; in the previous WHO classification, a tumor component (a particular growth pattern) was considered as a constituent only if it accounted for 10%, whereas the new classification recommends that it should be described in the diagnosis as soon as it reaches 5%. Clinical rationale for the new classification criteria For progressive NSCLC, it should be subdivided into more definite types, such as adenocarcinoma or squamous carcinoma, whenever possible. The rationale is as follows: ① adenocarcinoma or NSCLC-NOS should be tested for epidermal growth factor receptor (EGFR) mutation status, as it predicts EGFR-tyrosine kinase inhibitor (TKI) efficacy; ② adenocarcinoma is an effective and strong predictor of pemetrexed treatment compared to squamous carcinoma; ③ potentially fatal major bleeding can occur in patients with squamous carcinoma treated with bevacizumab.