Clathrin-mediated endocytosis (CME) is a ubiquitous internalization process in eukaryotic cells. It consists of the formation of an approximately 50-nm diameter vesicle out of a flat membrane. Genetics, biochemistry, and microscopy experiments performed in the last four decades have been instrumental to discover and characterize major endocytic proteins in yeast and mammals. However, due to the highly dynamic nature of the endocytic assembly and its small size, many questions remain unresolved: how are endocytic proteins organized spatially and dynamically? How are forces produced and how are their directions controlled? How do the biochemical activities of endocytic proteins and the membrane shape and mechanics regulate each other? These questions are virtually impossible to visualize or measure directly with conventional approaches but thanks to new quantitative biology methods, it is now possible to infer the mechanisms of endocytosis in exquisite detail. This book introduces quantitative microscopy and mathematical modeling approaches that have been used to count the copy number of endocytic proteins, infer their localization with nanometer precision, and infer molecular and physical mechanisms that are involved in the robust formation of endocytic vesicles.