This paper defends the view that metaphysical naturalism is a constitutive ontological principle of science in that the general empirical methods of science, such as observation, measurement and experiment, and thus the very production of empirical evidence, presuppose a no-supernature principle. It examines the consequences of metaphysical naturalism for the testability of supernatural claims, and it argues that explanations involving supernatural entities are pseudo-explanatory due to the many semantic and ontological problems of supernatural concepts. The paper also addresses the controversy about metaphysical versus methodological naturalism.