There are various receptor tyrosine kinase (TK)-targeted drugs that are currently used in the treatment of patients with non-small cell lung cancer (NSCLC). Among them, the epidermal growth factor receptor (EGFR) TK inhibitors (TKIs) are the most extensively studied. Receptor TKIs including EGFR TKIs have shown dramatic therapeutic efficacies in malignant tumors, which harbor activating mutations in the EGFR gene. However, within 1 or 2 years after treatment, patients harboring these mutations often develop resistance to TKI therapy. This review article is aimed at drawing attention to the fact that we must first understand how receptor TKI resistance is acquired to develop strategies for overcoming resistance to TKIs. Furthermore, an insight into the specific molecules or signaling pathways that mediate resistance is a key factor for understanding and overcoming acquired drug resistance. Finally, we present our views on the continuing battle against “drug resistance,” and provide further guidelines and strategies on how to minimize the development of drug-resistant tumors.