This book is about the nature of scientific theory. The central topicic of inquiry concerns how it is that theories are able to supply us with powerful and elegant explanations of puzzling phenomena that often confront the scientist and layman alike. It is argued that an answer to this question supplies us with an account of how theories achieve a variety of tasks such as the prediction and organization of data, including how they support a very important class of claims known int he literature as counterfactual conditionals. The book begins by presenting a critical survey of past, classic formulations of the nature of scientific theory which are promient in philosophy of sciences circles today. These include the doctrines of logical positivism, Hempel's Deductive-Nomological model of explanation, Hanson's gestalt approach to understanding and observation, Kuhn's sociology of science, and others. After presenting the reader with a critical examination of the above approaches to the nature of scientific theory, the author then presents his own views. His approach is essentially an ontological one. Ontology is usually characterized as the sudy of the nature of the most fundamental constituents of the universe. The major contention of the book is that theories are essentially deptictions of the nature of things, and that it is this feature which accounts for their ability to explain, predict and organize a vast array of data. In the tradition of more recent versions of scientific realism that have occured in the literature, the author attempts to show that the very confirmation of a tgheory depends on its ability to refer to the fundamental constituents of nature. It is argued that science can function only from an ontological point of view. In order to show this, the student is presented with a model of how theories are confirmed which is then cojoined with a model of the nature of scientific explanation. In so doing, the author ends up fostering a view of science which is rather controversial to twentieth-century philosophical tradition, namely that science is really metaphysics in disguise but a metaphysics which can ultimately be judged by empirical standards. Such an approch to science characterizes the modern-day scientist as an old-fashioned natural philosopher.