This article has two goals: a historical and a speculative one. The historical goal is to offer a coherent account of Spinoza’s view on mereological composition. The speculative goal is to show that Spinoza’s substance monism is distinct from versions of monism that are currently defended in metaphysics and that it deserves the attention of contemporary metaphysicians. Regarding the second goal, two versions of monism are currently defended and discussed in contemporary metaphysics: existence monism according to which there actually exists exactly one concrete entity; and priority monism that is famously defended by Jonathan Schaffer and according to which there exists exactly one fundamental concrete being, the cosmos, and several derivative concrete beings that are the parts of the cosmos. In this article, I argue that substance monism is neither an existence nor a priority monism because, while Spinoza’s monist is committed to the existence of a unique fundamental individual—the substance—and to the existence of several derivative individuals—the bodies or modes of extension—, Spinoza denies that the substance is mereologically complex. Regarding the first goal, the paper solves several interpretative puzzles by arguing that Spinoza distinguishes between three kinds of composition and that his talk of composition between the substance and its modes is by his own light not to be interpreted as literally true.