Control of cell number requires the coordinate regulation of cell proliferation and cell death. Studies in both the fly and mouse have identified the Hippo kinase pathway as a key signaling pathway that controls cell proliferation and apoptosis. Several studies have implicated the Hippo pathway in a variety of cancers. Recent studies have also revealed a role for the Hippo pathway in the control of cell fate decisions during development. In this review, we will cover the current model of Hippo signaling in development. We will explore the differences between the Hippo pathway in invertebrates and mammals, and focus on recent advances in understanding how this conserved pathway is regulated.