Erlotinib HCl (Tarceva; Genentech, Inc, South San Francisco, CA) is an orally available, highly selective, reversible inhibitor of epidermal growth factor receptor (HER1/EGFR) tyrosine kinase. Inhibition of tyrosine kinase activity prevents HER1/EGFR phosphorylation, the associated downstream signaling events, and may block tumorigenesis mediated by inappropriate HER1/EGFR signaling. In vitro and in vivo studies show that erlotinib has activity against human colorectal, head and neck, non-small cell lung, and pancreatic tumor cells. Recent preclinical studies suggest that erlotinib may also have activity against tumors that are dependent on HER2 activation for growth and/or survival. Preclinical studies have addressed the feasibility of using erlotinib in combination with various chemotherapeutic agents, radiotherapy, and targeted agents. Combining agents that have different mechanisms of action has the potential to improve efficacy and inhibit the development of resistance. For example, in preclinical studies, combining erlotinib with cisplatin, doxorubicin, gemcitabine, or low-dose paclitaxel has an additive effect on antitumor activity with no increase in toxicity. Preclinical data provide a strong rationale for investigating erlotinib in the clinical setting. However, additional studies are required to gain further insights into the processes that regulate or influence the antitumor activity of erlotinib.