The new World Health Organization (WHO) classification of lung tumours, published in 2001, updates the previous 1981 classification. This revised classification aims to improve reproducibility, clinical significance, and simplicity, minimizing the number of unclassifiable lesions. Key changes include the addition of pre-invasive lesions such as atypical adenomatous hyperplasia and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia. Adenocarcinoma is redefined, with bronchioloalveolar carcinoma restricted to noninvasive tumours. Large cell neuroendocrine carcinoma (LCNEC) is now recognized as a high-grade non-small cell carcinoma with neuroendocrine features. The classification also introduces a new class, carcinoma with pleomorphic, sarcomatoid, or sarcomatous elements. The classification is based on histological characteristics, primarily using light microscopy, though immunohistochemistry and electron microscopy are valuable for diagnosis. The classification includes various tumour types, such as squamous cell carcinoma, adenocarcinoma, large cell carcinoma, and neuroendocrine tumours. Adenocarcinoma is the most common histological subtype, with significant variations in prognosis based on subtype. Pre-invasive lesions like atypical adenomatous hyperplasia are considered precursors to adenocarcinoma, though not all cases of mucinous BAC have a defined pre-invasive lesion. The classification also defines basaloid carcinoma as a variant of large cell carcinoma, characterized by small cells with high mitotic activity. Neuroendocrine tumours are classified into typical, atypical, and high-grade categories, with LCNEC and small cell carcinoma (SCLC) having distinct prognostic implications. The classification emphasizes the importance of immunohistochemical markers, such as TTF-1, in differentiating lung adenocarcinoma from metastatic adenocarcinoma. The WHO/IASLC classification provides a comprehensive framework for lung tumour diagnosis and classification, aiming to improve consistency in patient treatment and support epidemiological and clinical studies. It includes detailed descriptions of various tumour types, their histological features, and prognostic significance. The classification is practical for surgical pathology laboratories and is supported by extensive research and clinical data.The new World Health Organization (WHO) classification of lung tumours, published in 2001, updates the previous 1981 classification. This revised classification aims to improve reproducibility, clinical significance, and simplicity, minimizing the number of unclassifiable lesions. Key changes include the addition of pre-invasive lesions such as atypical adenomatous hyperplasia and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia. Adenocarcinoma is redefined, with bronchioloalveolar carcinoma restricted to noninvasive tumours. Large cell neuroendocrine carcinoma (LCNEC) is now recognized as a high-grade non-small cell carcinoma with neuroendocrine features. The classification also introduces a new class, carcinoma with pleomorphic, sarcomatoid, or sarcomatous elements. The classification is based on histological characteristics, primarily using light microscopy, though immunohistochemistry and electron microscopy are valuable for diagnosis. The classification includes various tumour types, such as squamous cell carcinoma, adenocarcinoma, large cell carcinoma, and neuroendocrine tumours. Adenocarcinoma is the most common histological subtype, with significant variations in prognosis based on subtype. Pre-invasive lesions like atypical adenomatous hyperplasia are considered precursors to adenocarcinoma, though not all cases of mucinous BAC have a defined pre-invasive lesion. The classification also defines basaloid carcinoma as a variant of large cell carcinoma, characterized by small cells with high mitotic activity. Neuroendocrine tumours are classified into typical, atypical, and high-grade categories, with LCNEC and small cell carcinoma (SCLC) having distinct prognostic implications. The classification emphasizes the importance of immunohistochemical markers, such as TTF-1, in differentiating lung adenocarcinoma from metastatic adenocarcinoma. The WHO/IASLC classification provides a comprehensive framework for lung tumour diagnosis and classification, aiming to improve consistency in patient treatment and support epidemiological and clinical studies. It includes detailed descriptions of various tumour types, their histological features, and prognostic significance. The classification is practical for surgical pathology laboratories and is supported by extensive research and clinical data.