Introductions to Spinozas philosophy inevitably begin by invoking the great themes of Alain Badiou thought. Beyond the quite legitimate claims, it is central to Badious appreciation of Spinozas manifestation of his commitment to mathematics in the form of the mos geometrico that it involves a positive commitment to an approach to being. This chapter divides Badious elementary concepts in Being and Event into three categories with an eye to the way in which they appear in the work of his obscure precursor Spinoza. The first case has an explicit and fundamental agreement on the unique status of mathematics in relation to being. In the second category are those concepts that have obscure and implicit counterparts in Spinozas metaphysics. The third great ontological category that has no complement, explicit or otherwise, in the great pages of Spinoza, and it includes a concept of supreme importance for Badiou: the event.