This is a work in analytic metaphysics, which addresses a cluster of interrelated issues at the interface of mereology and persistence over time. In particular, it outlines a defence of a version of Endurance Theory according to which every enduring object is either a mereological simple or a mere sum of mereological simples. It includes, among other things, a proposal of a new way of framing the debate between Endurance Theory and Four-Dimensionalism, a defence of Endurance Theory over Four-Dimensionalism, arguments (...) against the existence of compound substances, and a defence of a traditional metaphysical atomism according to which all objects are ultimately made up of microscopic simples. (shrink)