The past decade has seen the elucidation of many of the events and processes responsible for receptor-mediated endocytosis. However, a fundamental question about the endocytic pathway remains unresolved: do early endosomes mature into late endosomes, or are these two distinct and pre-existing cellular organelles? General opinion tends to favour the former possibility, to the point where one poster session at the recent American Society for Cell Biology meeting* was entitled 'Maturation of Endosomes'. This article draws together new data arguing in favour of pre-existing early and late endosomes, between which transport occurs by vesicle budding and fusion.