The central aim of my thesis is to enquire into Spinoza's theory of the structure of the physical universe. It is generally accepted that from a scientific point of view Spinoza regarded the universe as consisting of particles in motion. My major concern is with the nature of these particles and what role they play in his cosmology. My basic method of enquiry is to consider, as far as this is possible, Spinoza's statements about the structure of matter as a scientific theory, a system of physics. Chapter Two is a brief survey of Spinoza's scientific activities, by way of providing background. In Chapter Three I explore the physical theory presented in Part Two of the Principles of Cartesian Philosophy with particular emphasis on the basic premisses of that theory, and the problems arising from those basic premisses. In Chapters Four, Five, and Six, the physical theory of the Ethics is discussed, with particular attention to Spinoza's theory of individuals and his ideas on motion. Chapter Seven is a fairly detailed discussion of the nature of the simplest bodies in Spinoza's system. Chapter Eight is a discussion of Spinoza's concept of the universe as a system composed of particles in motion, with particular emphasis on his ideas on the continuum. My major conclusion is that as far as scientific explanation is concerned, the Spino-zistic physics is very similar to the Cartesian physics as presented in the Principles of Cartesian Philosophy. This conclusion is based on five considerations: (1) the general character of the physics of the Ethics is quite compatible with the Cartesian physics with one apparent exception: I give an explanation of this apparent discrepancy; (2) Spinoza's deep concern with the problems of the continuum can only be explained if the basic premisses of his physics are the same as that of the Cartesian physics; (3) the same applies to his denial of the existence of the vacuum; (4) there is nothing that indicates that Spinoza's scientific method if radically different from that underlying the Principles of Cartesian Philosophy; (5) the one instance of Spinoza's outrightly condemning the Cartesian physics is based on a fundamental metaphysical issue and has no direct bearing on that physics qua physics. The major implication of my conclusion is that many of Spinoza's points of doctrine cannot be fully understood unless they are interpreted in the context of the Cartesian cosmology.