Substance (substantia, zelfstandigheid)’ is a key term of Spinoza’s philosophy. Like almost all of Spinoza’s philosophical vocabulary, Spinoza did not invent this term, which has a long history that can be traced back at least to Aristotle. Yet, Spinoza radicalized the traditional notion of substance and made a very powerful use of it by demonstrating – or at least attempting to demonstrate – that there is only one, unique substance – God (or Nature) – and that all other things are merely modes or states of God. In the first section, I examine Spinoza’s definitions of "substance" and "God" at the opening of the Ethics. In the second section, I study the properties of the fundamental binary relations pertaining to Spinoza’s substance: inherence, conception, and causation. The third section is dedicated to a clarification of Spinoza’s claim that God, the unique substance, is absolutely infinite. The fourth section studies the nature of Spinoza’s monism. It will discuss and criticize the interesting yet controversial views of Martial Gueroult, about the plurality of substances in the beginning of the Ethics and evaluate Spinoza’s kind of ontological monism. The fifth and final section explains the nature, reality, and manner of existence of modes.