Recent studies using electron microscopy, protein crystallography, classic biochemistry and novel live-cell imaging have provided numerous insights into the endocytic pathway, describing a dynamic system in which compartment morphology, molecular identity and the mechanics of cargo sorting are intimately connected. Current evidence supports a model of maturation in which the lipids, cargo proteins and Rab population at the endosome determine its competence to perform the functions of late endosomes, including the sorting of cargoes into lumenal vesicles and fusion with lysosomes.