Lung cancer is a malignancy that arises in the lung epithelium. It is clinically classified into small cell lung cancer and non-small cell lung cancer, which includes squamous cell carcinoma, adenocarcinoma, and large cell carcinoma. The main risk factors for lung cancer are smoking and exposure to secondhand smoke or other carcinogens. Signs and symptoms may include cough, coughing up blood, weight loss, chest pain, and shortness of breath. Diagnosis involves tests such as chest X-rays, CT scans, and biopsies. Treatment depends on the cancer type and stage but may include surgery, chemotherapy, radiation therapy, and lifestyle changes like quitting smoking.