The Hippo signaling pathway, first identified in Drosophila, is a conserved regulator of organ size. It involves a kinase cascade starting with the tumor suppressor Hippo (Mst1 and Mst2 in mammals) and leading to the oncoprotein Yki (YAP and TAZ in mammals), a transcriptional coactivator. This pathway integrates multiple upstream signals to dynamically regulate tissue homeostasis during development and physiology. In Drosophila, the core kinase cascade includes Warts (Wts), Salvador (Sav), Hippo (Hpo), and Mob-astumor-suppressor (Mats). These components form a kinase cascade where Hpo-Sav phosphorylates and activates Wts-Mats, which in turn phosphorylates and inactivates Yki. Yki functions as an oncogene, and its overexpression mimics loss of Hippo signaling. Genetic analysis shows Yki is downstream of Hpo, Sav, or Wts, and biochemical studies demonstrate that Wts directly phosphorylates and inactivates Yki in a Hpo-regulated manner. In Drosophila, additional tumor suppressors, including FERM domain proteins Merlin (Mer) and Expanded (Ex), protocadherins Fat (Ft) and Dachsous (Ds), CK1 family kinase Disc overgrown (Dco), WW and C2 domain-containing protein Kibra, and apical transmembrane protein Crumbs (Crb), have been implicated in regulating the Hippo pathway. These upstream regulators converge on the Hippo kinase cascade, and mutations in each gene lead to a relatively mild overgrowth phenotype, suggesting combinatorial or additive regulation. The Kibra-Ex-Mer complex is involved in regulating the Hippo kinase cascade. This complex physically associates with the Hpo-Sav complex and is required for membrane association of Hpo. Interaction between Kibra and Wts was also reported. The Kibra-Ex-Mer complex appears to interact with the Hippo kinase cascade through multiple protein-protein interactions, and these physical interactions are likely critical for bringing the Hippo kinase cassette to plasma membrane for activation. The tumor suppressor Mats is activated at the cell membrane. The transmembrane protein Fat (Ft) was the first transmembrane protein shown to impact Hippo signaling. Ft influences planar cell polarity (PCP) and is regulated by the steepness of the Ds gradient. The key downstream mediator of Ft action on the Hippo pathway is the unconventional myosin Dachs, which functions antagonistically to Ft and upstream of Wts. Dachs associates with Wts when overexpressed in Drosophila cell cultures and may function by promoting Wts proteolysis. Another protein that impacts Dachs activity is the palmitoyltransferase Approximated (App), which antagonizes Ft signaling by regulating the levels and subcellular localization of DachsThe Hippo signaling pathway, first identified in Drosophila, is a conserved regulator of organ size. It involves a kinase cascade starting with the tumor suppressor Hippo (Mst1 and Mst2 in mammals) and leading to the oncoprotein Yki (YAP and TAZ in mammals), a transcriptional coactivator. This pathway integrates multiple upstream signals to dynamically regulate tissue homeostasis during development and physiology. In Drosophila, the core kinase cascade includes Warts (Wts), Salvador (Sav), Hippo (Hpo), and Mob-astumor-suppressor (Mats). These components form a kinase cascade where Hpo-Sav phosphorylates and activates Wts-Mats, which in turn phosphorylates and inactivates Yki. Yki functions as an oncogene, and its overexpression mimics loss of Hippo signaling. Genetic analysis shows Yki is downstream of Hpo, Sav, or Wts, and biochemical studies demonstrate that Wts directly phosphorylates and inactivates Yki in a Hpo-regulated manner. In Drosophila, additional tumor suppressors, including FERM domain proteins Merlin (Mer) and Expanded (Ex), protocadherins Fat (Ft) and Dachsous (Ds), CK1 family kinase Disc overgrown (Dco), WW and C2 domain-containing protein Kibra, and apical transmembrane protein Crumbs (Crb), have been implicated in regulating the Hippo pathway. These upstream regulators converge on the Hippo kinase cascade, and mutations in each gene lead to a relatively mild overgrowth phenotype, suggesting combinatorial or additive regulation. The Kibra-Ex-Mer complex is involved in regulating the Hippo kinase cascade. This complex physically associates with the Hpo-Sav complex and is required for membrane association of Hpo. Interaction between Kibra and Wts was also reported. The Kibra-Ex-Mer complex appears to interact with the Hippo kinase cascade through multiple protein-protein interactions, and these physical interactions are likely critical for bringing the Hippo kinase cassette to plasma membrane for activation. The tumor suppressor Mats is activated at the cell membrane. The transmembrane protein Fat (Ft) was the first transmembrane protein shown to impact Hippo signaling. Ft influences planar cell polarity (PCP) and is regulated by the steepness of the Ds gradient. The key downstream mediator of Ft action on the Hippo pathway is the unconventional myosin Dachs, which functions antagonistically to Ft and upstream of Wts. Dachs associates with Wts when overexpressed in Drosophila cell cultures and may function by promoting Wts proteolysis. Another protein that impacts Dachs activity is the palmitoyltransferase Approximated (App), which antagonizes Ft signaling by regulating the levels and subcellular localization of Dachs