Non-small cell lung (NSCLC) cancer is responsible for approximately 85% of the 221,130 lung cancer cases in the US in 2011. Chemotherapy and chemotherapy with targeted monoclonal antibodies for patients with NSCLC remain modestly effective. Therefore, more effective targeted agents are needed for systemic therapy of different NSCLCs as well as the biomarkers associated with treatment benefit. The association between somatic mutations in the epidermal growth factor receptor (EGFR) and the clinical efficacy of the EGFR tyrosine kinase inhibitors, gefitinib and erlotinib, was discovered in 2004. Subsequent clinical studies done in Japan, China, East Asia, and Europe showed NSCLC patients with EGFR mutations treated with gefitinib or erlotinib have a 2-3 fold greater response and progression-free survival with less toxicity than those treated with platinum-based systemic chemotherapy. Further information is needed about patients with NSCLC and EGFR mutations treated with the different EGFR inhibitors in the US, Europe, and around the world. In addition, defining the mechanisms of resistance, identifying means of overcoming resistance to EGFR inhibitors, and improvements in the inhibitors are needed. The aims of the study are to prospectively validate the frequency and type of acquired resistance mutations and genomic changes arising in subjects with advanced NSCLC and somatic sensitizing mutations of EGFR treated with EGFR inhibitors. Studies of EGFR mutations in NSCLC patients treated with EGFR inhibitors around the world will define the relationship between EGFR mutations, response to treatment, progression-free survival, and survival in subjects with NSCLC treated with erlotinib and gefitinib in subjects with European ethnic background and those from East Asia. The experiments on different therapeutic approaches in lung cancer cell lines and models of acquired resistance to EGFR inhibitors will be done by studying tumors and tumor cell lines harvested or established from patients with clinical resistance to EGFR tyrosine kinase inhibitors. This will facilitate the development of more effective EGFR inhibitors either alone, or with other tyrosine kinase inhibitors and additional targeted agents.