The discovery of Anaplastic Lymphoma Kinase (ALK) by Stephan Morris and colleagues twenty years ago has led to an unprecedented opportunity and provided the basis for a novel and clinically powerful stratification of human cancers. The molecular and biological characterization of ALK and the recognition of alternative mechanisms of activation of the tyrosine kinase receptors have then set the basis for the development and the subsequent application of selective small molecules. These achievements have fostered a new era in oncology, and the result of this new avenue has drastically changed the expectation of many cancer patients. Here we review the mechanisms of ALK activation and the modalities that drive ALK pathogenesis.