This paper examines the nature and varieties of philosophical naturalism. A central question it pursues is whether there is any unifying conception of naturalism and, if so, whether it is substantive or methodological. Another question addressed is the extent to which naturalism is motivated by or depends on empiricism. The paper explores the connection between naturalism and scientific method---often taken as central in defining it---and critically discusses naturalistic positions in metaphysics (including philosophical theology), epistemology, and ethics. Given the ambitions of philosophical naturalism---which extend to construing philosophy itself as broadly empirical in the way that natural science is---and given some of the difficulties that confront naturalism, its success remains a matter of lively controversy.