Tumor biology is increasingly important when choosing the optimal therapy for patients with non-small cell lung cancer (NSCLC). A number of potential biomarkers is under investigation in the hope that it will be possible to identify markers that assist in the selection of patients for specific therapies in the future. Patients with an elevated DNA repair capacity, indicated by an increased tumoral expression of excision repair cross complementation group-1 (ERCC1) or ribonucleotid reductase subunit M1 (RRM1) may benefit less from cisplatin-based and gemcitabine-based chemotherapy, respectively. Overexpression of the cell cycle regulator p27 affects response to various anticancer drugs and increased levels of class III beta-Tubulin are associated with taxane resistance. Promising results so far suggest that customized therapy for individual patients with the help of predictive biomarkers is possible and it is likely that this strategy will improve treatment of NSCLC in the future.