The notion of validity in the social sciences is evolving and is influenced by philosophy of science, critiques of objectivity, and epistemological debates. Methodology for validation of the knowledge claims is diverse across different philosophies of science. In other words, definition and the way to establish of validity have evolved as epistemology of science changed. In this paper, I attempt to explain the nature of knowledge, outline the central premises of several philosophies of science (e.g., logical positivism, post positivism, scientific realism, postmodernism, and hermeneutics) and their positions on the issue of validity, provide a history of test validation from the beginning of educational and psychological testing until the present, with emphasize on changes in epistemology of knowledge. The concepts, methods, and processes of validation are central to educational and other social research, it is important to examine each epistemology that lie at the heart of both natural and social science. (Contains 1 table.)