This paper argues that ontological immanence involves but is not reducible to substance monism. Attending to immanence in Spinoza’s ontology, I provide a creative exegesis of the defining features of Spinoza’s immanent ontology, arguing that it recasts the concept of substance itself, from a term of transcendence and totalization to one of immanence and differentiation. In critical conversation with Deleuze’s influential reading, I identify five interconnected features which, taken together, elaborate Spinoza’s ontology of immanence: substance monism, univocity of attributes, immanent causality, the identification between G-d and Nature, and the status of finite modes as explications of substance rather than its extrinsic effects. I argue that, taken together, these features refashion the concept of substance, such that substance becomes not a term of totalization but rather one of ongoing production of diversity. Attending to the role of finite modes in this ontology, I emphasize the ways in which immanence can lend force to vital reconfigurations of ethical and political life: by defining beings and systems in terms of their capacities, which are augmented, diminished, or maintained depending on how they converge in relation.