This 2 arm study will assess the efficacy and safety of Tarceva plus gemcitabine, compared with gemcitabine alone, in the treatment of chemotherapy-naive patients with advanced non-small cell lung cancer. Patients will be randomized to receive either Tarceva 150mg po daily plus gemcitabine on days 1, 8, 15 and every 4 weeks subsequently, or with gemcitabine monotherapy. The anticipated time on study treatment is until disease progression, and the target sample size is 100-500 individuals.