In this chapter, a brief discussion of the lack of a general concept of knowledge on the one hand and on the other, the wealth of contributions to its understanding serves as an introduction to the multiple fields of knowledge studies. Three major areas are reviewed. First, the most prominent streams of the philosophy of knowledge, including positivism, conventionalism, postmodernism, critical theory, and pragmatism. Second, the most established sciences of knowledge are introduced, including anthropology, history, biology, psychology, economy, sociology, political science, knowledge and intellectual capital management and science and technology studies. This is complemented with some recent scientific developments beyond these disciplines. Third, alternative ways of knowing, including indigenous knowledge systems, are incorporated into the preliminary map of knowns and unknows about knowledge. Overall, this picture provides a diverse and complex perspective. The case for an integrated theory of knowledge is made.