A year has passed since mutations of the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) were discovered in patients with non–small cell lung cancer (NSCLC) who had dramatic clinical responses to treatment with gefitinib. Additional laboratory and clinical studies have provided further insight into the biological impact of EGFR mutations in cell culture experiments and in patients with NSCLC. In vitro characterizations of NSCLC cell lines and host cell lines transfected with these mutant and wild-type EGFR show that most cell lines with mutated EGFR are growth-inhibited by 10- to 100-fold lower concentrations of gefitinib and erlotinib compared with wild-type EGFR. NSCLC lines with mutations of the EGFR treated with concentrations of gefitinib and erlotinib that are achievable in the plasma undergo apoptosis rather than growth arrest. Retrospective studies of patients with NSCLC-treated gefitinib have reported a close association between EGFR mutations, increased chance of clinical response and longer survival. This review will provide information on the impact of EGFR mutations on gefitinib and erlotinib treatment by in vitro experiments, the outcome of NSCLC patients with these mutations when treated with gefitinib and erlotinib, and the subsets of patients with NSCLC in whom these mutations arise.