In this dissertation, I re-interpret Spinoza's parallelism theory in the Ethics according to an original ontological, dualistic and idealistic perspective. The main thesis I put forward is that parallelism, and the dualism it harbours, is the only possible mediation between Spinoza's monism and his pluralism, i.e. between the unique substance and the 'infinite things in infinite modes' that follow from it. Second, I argue that the theory of parallelism is both coherent and unitary and no fracture should be viewed between Proposition 7 of Part II of the Ethics and its following Scholium. This interpretative strategy is original as it takes a route that is generally neglected by Spinoza scholarship, and thus is in contrast to those interpretations that see an unsurmountable contradiction in Spinoza's parallelism or that distinguish an ontological variant from an epistemological variant of it. Moreover, my interpretation of parallelism originally clarifies some obscure passages in later parts of the Ethics, such as the Spanish poet anecdote and the eternal mind doctrine. My interpretation is the result of a full-fledged idealistic interpretation that is based on the pronounced pre-eminence of the attribute of thought in Spinoza's system and that leads me to originally re-interpret some key doctrines of Spinoza's metaphysics, such as the conceptual independence of attributes that can be found in Proposition 10 of Part I. Additionally, in my dissertation, I draw some theoretical and historico-philosophical comparisons between Spinoza and past thinkers or his contemporaries (Scholastic authors, Descartes, Leibniz) as they clearly indicate that parallelism is an original and unprecedented thesis in the history of philosophy.