Epidermal growth factor receptor (EGFR) and HER2 are cell surface receptor tyrosine kinases (TKs) that transduce growth signals through dimerization with HER family receptors. The heterodimerization of EGFR with HER2 induces a more potent activation of EGFR TK than does EGFR homodimerization. When tumor cells overexpress both EGFR and HER2, they exhibit aggressive tumor cell growth, owing to the increased potential for EGFR/HER2 heterodimerization and signaling. Gefitinib and erlotinib are EGFR TK inhibitors (EGFR TKIs) and have antitumor activity in 8–18% of patients with advanced non-small-cell lung cancer (NSCLC). Certain patient subsets are particularly responsive to EGFR TKIs. Analyses of biomarkers from patients in clinical studies of EGFR TKIs show correlations between objective tumor response and EGFR overexpression, as detected by immunohistochemistry and increased gene copy number measured by fluorescence in situ hybridization analysis. Furthermore, NSCLC tumors that overexpress both EGFR and HER2 are more sensitive to EGFR TKIs than are tumors that overexpress EGFR but are HER2 negative. Therefore, the measurement of EGFR and HER2 protein expression and the gene copy number in NSCLC tumors may have a prognostic value in NSCLC and a predictive value for identifying patients likely to benefit from an EGFR TKI. These considerations suggest that the simultaneous inhibition of EGFR and HER2 may warrant further study in patients with NSCLC.