Clathrin-mediated endocytosis (CME) is the best-studied pathway by which cells selectively internalize molecules from the plasma membrane and surrounding environment. We study this process by live-cell microscopy in yeast and mammalian cells. The yeast studies have revealed a regular sequence of events necessary for endocytic vesicle formation involving some 60 proteins, which induce a highly choreographed series of changes in membrane geometry, ultimately resulting in scission and vesicle release. To analyze endocytic dynamics in mammalian cells in which endogenous protein stoichiometry is preserved, we have used genome editing for the clathrin light chain A and dynamin-2 genomic loci and generated stem cells expressing fluorescent protein fusions from each locus. These cell lines are being used to study actin assembly at endocytic sites and tomake 3D organoids in culture. Use nano-patterned substrates, we are actively studying roles for membrane curvature in endocytic dynamics. At the same time, studies in yeast cells have recently focused on discovery of regulatory mechanisms for insuring theproper order and timing of events in the endocytic pathway and how actin assembly at endocytic sites is regulated. Studying the yeast and mammalian systems in parallel is allowing us to translate what is learned from one system to the other.