Open randomized multicenter phase II study in patiënts in need of 2nd line treatment for advanced/metastatic NSCLC. Efficacy and safety of monotherapy with erlotinib will be compared with combination therapy of erlotinib and chemotherapy. In recent studies it was established that pemetrexed activity is more pronounced in non-squamous NSCLC in comparison to squamous cell carcinoma. Therefore in patients with non-squamous carcinoma pemetrexed will be used. As in second line treatment of NSCLC docetaxel is registered also for usage in patients with squamous cell carcinoma, docetaxel will be used in patients with squamous histology.