The goal of our research is to understand how endocytosis affects cell signaling in animal development. Endocytosis is a process characterized by the internalization of extra-cellular fluid, lipid, and membrane proteins through vesicular transport. Until recently, the role of endocytosis in signaling was mostly thought as a "turning-off" mechanism, that it is required for cells to internalize and degrade activated receptor molecules from cell surfaces. However, emerging evidences from several systems have clearly suggested that endocytosis has more elaborate roles in cell signaling. To understand this process further, we have focused on dissecting the roles of clathrin-mediated endocytosis in cell-cell signaling in Drosophila. Using a combination of genetics, cell biological, and ultra-structural analysis, we are characterizing mutations in several genes required for clathrin-mediated endocytosis. One of our mutants exhibits defects in the Notch signaling cascade, an essential pathway for many aspects of metazoan development. These data demonstrate that endocytosis is much more than cells "drinking" and "eating", but can directly regulate the activities of signaling molecules important for cell-cell communication.