Tyrosine kinase inhibitors directed against the epidermal growth factor receptor (EGFR) are the first molecular-targeted agents to be approved in the US and other countries for the treatment of advanced non-small-cell lung cancer after failure of chemotherapy. Some patient characteristics, such as never-smoking, female gender, East Asian origin, adenocarcinoma histology, and bronchioloalveolar subtype, are associated with a greater benefit from treatment with EGFR inhibitors. Recently, studies have identified gene mutations targeting the kinase domain of the EGFR that are related to the response to inhibitors. Most EGFR mutations predict a higher benefit from treatment compared with wild-type receptors and are correlated with clinical features related to better outcome; some EGFR mutations, however, confer drug resistance. The analysis of material usually available from lung cancer patients, using techniques such as direct sequencing to determine EGFR mutational status, can be technically challenging. In this regard, high EGFR copy number and EGFR protein detected by immunohistochemistry can also be used to select those patients who would benefit from treatment. Prospective validation of biological and clinical markers of sensitivity needs to be performed.