Several decades of cancer research have revealed a pivotal role for tyrosine kinases as key regulators of signaling pathways, controlling cell growth and differentiation. Deregulation of tyrosine kinase-mediated signaling occurs frequently in cancer and is believed to drive the initiation and progression of disease. Chromosomal rearrangements involving the tyrosine kinase anaplastic lymphoma kinase (ALK) occur in a variety of human malignancies including non-small cell lung cancer (NSCLC), anaplastic large cell lymphomas, and inflammatory myofibroblastic tumors. The aberrant activation of ALK signaling leads to "oncogene addiction" and marked sensitivity to ALK inhibitors such as crizotinib (PF-02341066). This review focuses on ALK rearrangements in NSCLC, starting with the discovery of the EML4-ALK fusion oncogene, and culminating in the recent validation of ALK as a therapeutic target in patients with ALK-rearranged NSCLC. Current efforts seek to expand the role of ALK kinase inhibition in lung and other cancers and to address the molecular basis for the development of resistance.