The anaplastic lymphoma kinase (ALK) fusion gene is a key oncogenic driver in a subset of patients with advanced non-small cell lung cancer (NSCLC). Oncogenic fusion genes, including echinoderm microtubule-associated protein-like 4 (EML4) and ALK, have been detected in approximately 2-7 % of NSCLC patients. Fluorescence in situ hybridization (FISH) is the recommended method for detecting ALK gene rearrangement. EML4-ALK fusion genes define a molecular subset of NSCLC with distinct clinical characteristic (lung adenocarcinoma, never or former smoker, usually mutually exclusive with EGFR mutations). Crizotinib (PF-02341066) is an orally bioavailable, ATP-competitive, small molecule inhibitor of both the receptor tyrosine kinases ALK and c-MET (hepatocyte growth factor receptor). Crizotinib has been shown to yield important clinical benefit such as objective response rate, progression-free survival (PFS), and anticipated improvements in quality of life when used in pretreated patients with advanced NSCLC harboring EML4-ALK gene rearrangement. Preliminary phase II data suggested that crizotinib is safe and well tolerated with rapid and robust antitumor activity. A phase III randomized trial in a second-line setting showed response rate and PFS (primary study endpoint) advantage for crizotinib as compared to second-line chemotherapy. Treatment-related adverse events, predominantly restricted to the gastrointestinal and visual systems, are generally self-limiting or easily managed. Crizotinib is a new standard of care for patients with advanced, ALK-positive, NSCLC. In this review, we will discuss the discovery of ALK rearrangements, the clinical epidemiology of lung cancer driven by ALK, the clinical data for ALK-targeted therapy in NSCLC, and ongoing ALK inhibitor-based clinical trials.