Mutations in the tumor suppressor gene LKB1 are important in hereditary Peutz–Jeghers syndrome, as well as in sporadic cancers including lung and cervical cancer. LKB1 is a kinase-activating kinase, and a number of LKB1-dependent phosphorylation cascades regulate fundamental cellular and organismal processes in at least metabolism, polarity, cytoskeleton organization, and proliferation. Conditional targeting approaches are beginning to demonstrate the relevance and specificity of these signaling pathways in development and homeostasis of multiple organs. More than one of the pathways also appear to contribute to tumor growth following Lkb1 deficiencies based on a number of mouse tumor models. Lkb1-dependent activation of AMPK and subsequent inactivation of mammalian target of rapamycin signaling are implicated in several of the models, and other less well characterized pathways are also involved. Conditional targeting studies of Lkb1 also point an important role of LKB1 in epithelial–mesenchymal interactions, significantly expanding knowledge on the relevance of LKB1 in human disease.