The treatment of advanced non-small cell lung cancer (NSCLC) by therapies targeting the epidermal growth factor receptor (EGFR) pathway represents one of the most important advances in thoracic oncology. Reversible EGFR tyrosine kinase inhibitors (TKIs), like gefitinib and erlotinib, are able to achieve dramatic responses in a subset of patients. However, most patients treated with TKIs eventually develop resistance against these drugs. Here we review the physiology and pathology of EGFR activation in NSCLC, the clinical experience with TKIs, the mechanisms of resistance against TKIs, and discuss various approaches to treat resistance against TKIs.